My campaign is centered around a crime syndicate but I need ideas for leaders but all have a backstory personality and agenda.

i have a few ideas I just want to see if y’all’s are good too.

The mayor who uses the thieves to strategically keep crime high to justify his occasional crackdowns. 

An obscenely rich person who steals just for kicks, just to see if they can. Once they have what they’re after, they typically throw it in the river — or occasionally plant it on a rival.

The cult (there’s always a cult) who is after a macguffin so they can perform their ritual, which likely involves meddling in forces they don’t understand. But to throw off suspicion, they steal other random things so it’s harder to figure out their true goal.

A Robin Hood style group that steals from the rich to give to the poor.

I like the idea of a cultist leader who is in the syndicate for a specific treasure. So maybe like a sort of psychotic killer who is in it for their patron.

My personal idea is a doppelganger. Gotta have a doppelganger.

Rakshasa/other devil that is disguised and is in the syndicate to create strife and corrupt souls for use on the war in Avernus. 

Assassin that no one knows about that stalks the streets every night killing at will, typically enemies of the syndicate and guards that try to stop them.

An evil alchemist brewing a deadly gas to unleash on the city that will mind control everyone (but have the unintended side effect of slowly melting people's brains). They were told that they were crazy/mad scientist and they want revenge on those people, and to prove themself.

A cloistered bureaucrat that technically isn't doing anything against the law  by exploiting the rules and extorting money.

Crime lords are often not Rogues in literature. They are Fighters or casters or just non-class business people.

If you need a group, make a group that has every class. If you are the GM though, class doesn't matter.

LOVE THE RAKSHASA IDEA

The standard for a crime group is organised crime, which typically would mean money laundering, non-petty theft, and extortion (protection).

The beauty of a fantasy setting, however, is that you can expand the scope of the materials. Are they stealing mundane items? Gold? Dreams? Secrets? Magic? Smuggling animals into the city? Monsters? Are they replacing mid-ranking people with doppelgangers to facilitate a legally-binding coup (vote of no confidence to remove non-doppelgangers from the government)? Have they already done so, to suppress stricter controls on their chosen crimes?

You could have someone who sells energy drinks to the mages guild, with a bottle-return policy that makes the mages feel like they're cheating the system for how cheap they get the drink, but the bottles are actually siphoning first-level spell slots from the mages - the drink is just sugar and water. They then use the magic they have stolen to make & sell high-quality hallucinogens to rich aristocrats who seek to liven up their parties. The side-effect of this is that more people are seeking aid for magical maladies from the mages guild, who are working long hours to fix them. Fortunately, there's a new flavour of energy drink available...

Look for (and design) exploitable loops where unscrupulous people can make money. Using urchins to steal, because they are expendable and your crime lord will keep getting richer. Having a captive false hydra that the boss feeds people to who displease her - so everyone loves her, because no-one remembers anyone who doesn't. Smuggling in monsters whilst also running a monster removal service. A gang of druids running a pet shop. A Wizard, doing the same with trained imps he polymorphs into the cutest puppies.

One of my main plot lines is monster smuggling and I am planning to add a doppelgänger conspiracy too!

If you want some sort of big twist for the party, you could lay the foundations that the syndicate has placed doppelgangers in the government. After much deduction and confusion, the party will likely be arrested for assault, where they will eventually discover that the doppelgangers have been put in background positions - servants and the like - to first frame the non-doppelgangers for theft, and then the great heist will happen once all the serving staff are doppelgangers - the servants will all just suddenly disappear with all the loose gold and silverware and embroidery and such in tow, never to be seen again!

Slave traders are another common trope.

It's fairly rare for crime bosses to be (currently) thieves, because that's basically the job of lower ranks. Stealing also isn't typically the actual focus of criminal organizations (the exact focus depends on what's actually illegal, but typically you're looking at protection rackets, smuggling, trade in illegal products, and providing illegal services), though they may be involved in fencing stolen goods.

What I mean by not just thieves is that they have other agendas besides profit for themselves such as power or service to a god or demon.

Quote from ThorukDuckSlayer >>

If you want some sort of big twist for the party, you could lay the foundations that the syndicate has placed doppelgangers in the government. After much deduction and confusion, the party will likely be arrested for assault, where they will eventually discover that the doppelgangers have been put in background positions - servants and the like - to first frame the non-doppelgangers for theft, and then the great heist will happen once all the serving staff are doppelgangers - the servants will all just suddenly disappear with all the loose gold and silverware and embroidery and such in tow, never to be seen again!

 That is a great idea!!! I will definetely use this.

Quote from Furrybeardedmonk_198654 >>

What I mean by not just thieves is that they have other agendas besides profit for themselves such as power or service to a god or demon.

Plenty of organizations decide that crime is an excellent way to fund their activities. It's most often groups that are already illegal, such as rebel groups, because if you're wanted for one thing already, being wanted for two isn't that much of a stretch.

I mean look at Bill the butcher, Al Capone, lucky Luciano, none of them were theives. 

They could be business owners, fight fixers, import export tradesmen, they could head the smiths guild or the teamsters, not actually specialize in thriving, but in reputable transport and hauling. They could run a livestock auction or be in textile trades, easy money-ruch industries where obfuscating ill-gotten gains would be as simple as a few additional bills of lading or sale. 

Here what is what I used for a medium-length crime-fighting campaign:

Boss: Yuanti high priestess, she provides illusory cover for their base, provides deadly venom to assassins, and charms politicians in the city to her will. She is using the crime syndicate to find those worthy of being transformed into a new generation of Yuanti.

Inner Circle : are at various stages of Yuanti transformation, includes a Dragonborn Conquest Paladin who has been promised help to avenge the destruction of his clan, an orphan Lizardfolk raised by monks who sees the Boss as a mother figure, and a Tabaxi assassin who is tired of killing for money and wants a higher purpose to their murder.

Sub Bosses :
the last heir to an impoverished elven noble family who thinks they can use the syndicate for their own ends of becoming the local ruler. He engages in all kinds of business with no qualms of including smuggling, slavery, or other illegal practices so long as it earns a profit.

A doppleganger abandoned by their mage-creator who had no where else to go. Part private-eye, part honey-trap, part extortionist, they find out everyone secrets and sell them to the highest bidder.

A dwarven family who have been thugs for generations since being kicked out of their home underground city for being petty thieves. They run a large club full of high-stakes gambling, illegal fighting pits, and a drug den.

A disguised hag who acts as herbalist/healer, lawyer, and banker for the syndicate who ensnares the desperate in debts and contracts which are used to force them to act in the interest of the syndicate.

I think the fun part of building out a network of crime is you can really work it one of two ways.  

1) You can make it about resources.  While the mobs of the world were mostly there to make a lot of money, it can be a specific resource as well.  Human capital, monster capital, a specific finite resource that will go up in value, machines/magic that can be tied up in either blueprints or owning the only copies of the books known to exist.  All of these are potential interesting bits (the human/monster angle has some potential hurdles to clear with a group, you can do it without making it extremely dark, but there are some basic rights that some players won't like to see crossed).  I think my favorite remains the specific resource, because it allows you to send the group on an adventure to find the resource or to intercept it, etc.  

2) You can make it more of a land/turf war, where it's centered around ideology or families of crime, where their home city or area is fairly safe, but the borders that shift have cities that are ravaged by violence.  You can enter the group into one of the factions, sell it as the righteous cause, then throw curve-balls, or your party can be neutral to it all and decide to try to disrupt all of the organizations, which will then create problems for the group when all three see their intent and come after them.

A doppelganger can be included in either of those formats, either as part of one of the factions involved (mob or turf family) or perhaps as a neutral party that the group might have to contend with depending on what he is in it for.  Is he following the group at some point to try to capture one of them and assume their form?  Is he playing the part of someone who has been swindled, sending the group away while he goes on a crime spree?  Or is he conning the family he is working for in the mob/gang situation and is set to finish his ruse and make off with the goods?

If you want to mirror it off the real world situations in which gangs or mafias or etc rose up, consider the following reasons:

A) Lack of an established and capabale government - Perhaps the local lord of the land over the course of the last x amount of years has failed to properly punish those who commit crime, or has an in with the crime that is going on.  Organized crime might come in under the more heroic pretense of "we'll do what they wont do".  They'll grow in prominence, get intertwined more and more with the workings of the land, and use the influence in ways that might not benefit the land in the same way that their cause might have initially began.  

B) Species/Group divides - nationalism is a reason why some groups, specifically those that are not in the majority, will band together to combat perceived injustices whether they are legitimate (which they absolutely are in some cases) or sometimes stretched to accomplish a goal (also absolutely true in some cases).

C) Limited/Prohibition - Having something someone else doesn't have and wants can lead to a lot of control or power.  I've covered this enough above. 

The individual backgrounds can be that people seek protection, people seek brotherhood and may see it as such, they might have needed money and the traditional lenders wouldn't lend like the organized crime would, and now they have to pay their debts, some might be born into the life.  Very easy in my personal opinion to take a bunch of these different angles, create personalities that would fit with those ideals, and then create the backgrounds for the NPC's you need to fit the role based on that.

The detective agency is a front for a crime syndicate who uses it to sniff out the other criminals

A crime syndicate could be a front for a hostile foreign nation that seeks to undermine the local government.