I'm starting a campaign and there is a player who thinks she wants to make a gun-based character. Is there a way to make homebrew weapons, to allow for greater customization?

Yes theres literally a homebrew section for this website.

There's nothing for weapons in the homebrew, the closest thing there is is magic items.

Yeah, you have use the magic items homebrew tools to do it, and there are limitations: you can’t change a weapons damage dice, but you can change type and other properties.  So you have to start with a weapon that uses the right damage dice and work from there.  Apparently WOTC didn’t want them allowing for home brewing weapons initially.  I think that has changed, but there’s a ton of improvements and bug fixes DDB wants to do, plus WOTC keeps throwing new mechanics at them, so they haven’t had a chance to revamp and expand the homebrew system.

You can create custom actions on the sheet itself to represent the attack using that weapon. You can create a custom item in the inventory for recording whether it's equipped, the weight, and such as.

If you detail what weapon you're looking to homebrew I can provide the steps you need for creating it on your sheet.

The pro is that you can homebrew almost any sort of weapon as you want it to be used by the character. The con is that this is only effective on your sheet and cannot be transferred to other players - it's basically the equivalent of writing it out as if you had a pen and paper sheet, except it can autocalculate things for you.

Why is there no way to make a "new" weapon (or armor for that matter)? "Why do I have to take an existing weapon/armor and modify it as a magic item?

I just want to make a Plumbata (like a larger version of the "dart") which when thrown allows for greater distance and damage. 

I love the tools offered here but damn there are some weird things that are omitted for no good reason. If you have a whole homebrew section the developers should have expected users of the tools to want to make non magic items... and provided that option.

Quote from Cyb3rM1nd >>

You can create custom actions on the sheet itself to represent the attack using that weapon. You can create a custom item in the inventory for recording whether it's equipped, the weight, and such as.

If you detail what weapon you're looking to homebrew I can provide the steps you need for creating it on your sheet.

The pro is that you can homebrew almost any sort of weapon as you want it to be used by the character. The con is that this is only effective on your sheet and cannot be transferred to other players - it's basically the equivalent of writing it out as if you had a pen and paper sheet, except it can autocalculate things for you.

 Thanks for the tips! 

I just want a non-magical katana that is versatile 1d6-1d10, is not heavy, not light, is a martial weapon, and finesse 

I would bump this for continued relevance. I've been trying to make a 2d4 weapon and now find that I cannot. I may be able to make the item have a spell attack for the right effect and just deal with it for now.

Quote from Dirjiin >>

I just want a non-magical katana that is versatile 1d6-1d10, is not heavy, not light, is a martial weapon, and finesse 

 You can actually do that as it stands with the tools at hand.
Since you can add / remove weapon properties when creating a magic item, I'd just make it a Longsword + Finesse.
If you're okay with making it a 1d8 one handed, that is. 
Otherwise, I'd just make 2 different weapons, call the first Katana (2-handed) and make it a Longsword with Finesse, make a second item and call it Katana (1-handed) and remove light from Short Sword. Since the flavor of "damage type" doesn't really factor into things mechanically on the website, I typically ignore it for crafting custom weapons.
Gotta give a little in the "Homebrew-o-sphere" imo.

The reason we cannot homebrew basic equipment is because of this entry (RAW is to find the closest weapon of the list and reskin it):

It’s worth noting that the katana is the “dai” part of the classic pair of Japanese swords known as a “Daishō.” Daishō is actually a compound word that literally translates as “Big-Little,” describing the two swords in the pair. The “shō” part of Daishō is derived from the word “shōtō” which translates as “shortsword,” and the “dai” part is derived from the word “daitō” which literally translates as “Longsword.”

Frankly, it makes no sense that a Japanese Longsword should do less damage when wielded one-handed than a European Longsword would, so just let it be 1d8(versatile 1d10) and call it a day.

As to any (false) notion that a katana would have finesse, it would not. A European longsword, has a grip shaped hilt and a pommel to make it easier to maintain one’s grip against the centrifugal force of the mass over the the arc of the swing. It is designed to make it easier to swing and recover and readjust edge alignments quickly. It also has a flexible blade meaning it can find its alignment easier when it impacts. By comparison, a Japanese longsword has a strait haft, no pommel, and a very inflexible blade. All of that means it takes more strength to maintain one’s grip, and requires more power to push through a target if the attacker’s edge alignment was off. By all rights, a European longsword should have far more cause to claim the finesse property, the Japanese longsword, aka katana… not so much.

Finesse because it's magical I can see. Otherwise makes no sense.

As stated already, just use a longsword because it is the closest thing to what you want.

Yeah just found this out because I was hoping to homebrew a war spear for future use.  Since best guess the spear in game would be some kind of hunter's spear or something and historically spears could range in length from like 5 or 6 feet to 8 or even 10 feet with a military version leaning towards the latter.  I was thinking versatile with both using higher dice than the simple version, plus reach.

Using custom actions and custom items directly on the sheet can really help with this. I made a video going over this as well as other sheet customisations. It has chapters if you just want to skip to the part specifically for custom actions/items.

I think being able to create actual homebrew weapons would be great because then you'd be able to use it with features like the Artificer's Infusions. I don't think you can do that with custom actions

Sometimes one would like to make a special weapon with specific stats not found in the small list of existing weapons.
If I want a 1d20 weapon (incredibly randomized damage with high potential) I can't just go to homebrew tab and make a 1d20 damage like one would expect to do in a HOMEBREW TAB, and cannot modify the damage type or properties as well, the only ways are making a custom attack with custom item (The best option but is tedious to make), create a magical item with a template (crap), or limited modifications on existing weapons in the character sheet (also crap but a bit better, being most of my custom weapons I have).

>For example, I wanted a magical rifle (Wanted a loading, heavy, 2 handed, ranged, 1d20 force damage) but I cannot create it, I'll be using a modified Hunting rifle for now as it is the closest I can find... (2d10 piercing) just equip the rifle and change the name, but cannot change damage type unfortunately.

is there a work around to accurately make a khopesh which is essentially a sickle shaped longsword meant to grapple weapons shields or appendages I would imagine needing it to have finesse for the strange uses outside of regular strength attacks. And if I’m not mistaken they would also fall under veracitile in its abilities too.

The “grappling shields, weapons, and appendages” part would be flavor used during combat since axes and flails were designed to do the same things and there’s no special rules to govern that type of stuff. You could always just take a Longsword and add the finesse property to it, but that would be unbalanced from a game design standpoint compared to other weapons. When I added Khopesh to my game I instead started with a scimitar and added the versatile property.

how do i add a homebrew weapon i already made

Quote from BoomTrouserstheexplosive >>

how do i add a homebrew weapon i already made

 
Same way you add any other weapon. Just ensure you have Homebrew toggled on (home tab of character builder).