Currently playing a lvl 5 goo tomelock. I had in mind to level pure warlock, however lately I've been thinking about a little multiclass into something else. A couple of thoughts came to mind.

First, I was thinking of dipping 3 levels into sorcerer for the extra cantrips and spell slots. Grabbing favored soul would give me access to the cleric spells and sorc spells as well for a bit of flexibility.

My other thought was to dip 3 into lore bard to become a bit of a skill monkey, gaining expertise in say perception and deception to really have some fun. Plus some spell slots to play around with. 

I'm hesitant to go further than 3 into another class because I'm still into the idea of my 9th lvl mystic arcanum. 

Or maybe I shouod stick with just straight lock? Thoughts? Suggestions? Its worth noting I am in a 3 player party with a life cleric and a samurai. I'm really the only ranged attacker as the cleric tends to get up in melee to support the samurai.

If you stay a straight Warlock at 6th level you get Entropic Ward which works for ranged and melee attacks and at 7th level you get 4th level spells. Giving those up, especially Black Tentacles is pretty hard to do. I don’t think I can delay those two for extra spell slots or skills. 

Both are good options for increasing the warlock's flexibility both in and out of combat.

Tim makes some good points too though.

Multi-classing is rarely if ever a good practice to indulge in.  Delaying class features, especially those of a Warlock, means you will be behind the curve in most metrics. I would suggest playing your chosen class well and creatively and you will be fine, better in fact than if you decide to adulterate it by multi-classing. 

Quote from LBSchoener >>

Multi-classing is rarely if ever a good practice to indulge in.  Delaying class features, especially those of a Warlock, means you will be behind the curve in most metrics. I would suggest playing your chosen class well and creatively and you will be fine, better in fact than if you decide to adulterate it by multi-classing. 

 "Most metrics" is the tricky part here, isn't it? The Warlock class is good at some things, not so good at others, and multi-classing is one way to gain what the Warlock lacks. But if another class is better at doing something you want your character to do, that raises the question of why you're playing a Warlock in the first place.

I tend to agree with you. The Warlock is the most customizable class in all of 5E, so multi-classing is my last option when I need to add new capabilities. There's nearly always a racial ability, a feat, an invocation, or a spell that will do. Setting back my primary class progression is too high a price to pay when there are other options available.

Is your campaign expected to go all the way to level 20 (or at least level 17?) If so, you're likely better off waiting until later to multiclass, if at all. If not, then depending on which Warlock features you're going to miss out on, it might be worthwhile to multiclass at some point, maybe past level 9, if the upcoming warlock features look like they won't fare you any better than the dip.

That said though, I get the impression you're largely wanting to dip just to get some extra spell slots; right now you only have two, and won't get your third until level 11, and if you're trying to play your warlock very magic-heavy then you're probably finding you don't really have enough slots to do that. If that's one of your goals then maybe just a single level in another class for now might be a better option than 3 levels (and you could always grab more later.) It wouldn't fix your issue entirely--you might still want to grab Magic Initiate and spell-dealing magic items to supplement your class spells--but you'd have two more spell slots to make things a little easier.

My group is getting ready to start a new campaign, and I am going to play a celestial warlock.  two spell slots simply doesn't get it done for me.  I was going to dip a level of divine sorc, but I'm really starting to lean towards a level of bard to pick up some extra spell slots.

Try playing with those 2 slots first. If you get short rests they’re not as big of an issue as it looks at first, especially with everything else that warlocks can do. 

Quote from crzyhawk >>

My group is getting ready to start a new campaign, and I am going to play a celestial warlock.  two spell slots simply doesn't get it done for me.  I was going to dip a level of divine sorc, but I'm really starting to lean towards a level of bard to pick up some extra spell slots.

So why play a Warlock?

Celestial warlock has a lot of nice things going for it.  I just want a few more options than 2 spells slots until level 11 has to offer.  A level of bard gives me a couple of faerie fires to throw out and buff my party while I warlock the rest of the time.

I've played a sorlock and a cleric previously, so I've got a pretty good idea of what I am signing up for.  Pure warlock just doesn't interest me much.  Multi-class warlock interests me very much.  So why wouldn't I multi-class?  I lost more picking up 2 levels of warlock in my sorcerer than I am losing by picking up one level of bard in my warlock.

I have a level 5 Hex/Tome Warlock. My next level will be Sorcerer (Shadow). Probably more Warlock after that - I don't like to commit too far in advance.