Well, a quick point and then some recommendations. Classes and guilds are the same with one minor difference, when you join. I'm going to assume, for now, that you want the ability to leave guilds. Clarification on this point is necessary: what, to you, defines an lpmud guild as compared to a class?
And I don't have access to all of these at the moment to check which you've played.
I'm going to assume you've heard of discworld and batmud. If I recall correctly, they both call their classes guilds--of them, however, discworld is the closest to rom-style classes (you only ever get one, and your choice is final).
Batmud has something slightly different: you have "free levels" and can join any number of guilds in your background. You generally pick one of the main ones at a lower level, but you might branch off to become an ice mage or a warrior that tames animals or an assassin with poison. It's kind of hard to explain it. They're not expantion packs, however, each one is like having an additional class. Choices, on this one, are perminent as well.
Both of those are also pretty much the biggest lpmuds still around--they rival even 3k--and you've probably played them.
If you like role-playing, there's dartmud. Guilds don't exist mechanically beyond something that says you've joined it (if that? I really can't remember). Most of what you get from guilds comes from other players; spells are in books and you can only learn it if you have access to the book, players can teach you skills, give you services...The only hardcoded limit here is, if you learn a lot of fighting, your learning rate for magic becomes slower and vice versa--this is levelless, and there is crafting, and you could choose not to learn either if you wanted. Obviously, only try this if you like rp, as guilds don't have too many consequences beyond that (But that is a big one, given that it's an rp mud).
Lastly, lost souls--on the one hand I love it to death, on the other I hate it to death. Lost souls can be, emphasis on can, extremely brutal and hard, but if you're looking for unique guilds, you're not going to beat it. There's the erision liberation front (call on gods from just about every pantheon ever, fictional or not--don't make them mad or they might squash you), the aligned (channel the forces of order in a lot of odd ways), the travelers (you get challenges with an objective and an obsticle: exploration while blind is an extreme example of this), and the ringwielders which most players basically call make-your-own guild (wield a ring of power from a sauron expie). Choose a guild and any number of compatible non-guild affiliations with all sorts of benefits (hop across the world, remove need to eat)...
So why do I say I hate it to death. Well, if you don't know what you're doing, you will die, a lot. On lost souls, however, death is one of the better things that can happen to you--as long as you make sure you still have lives--and there's no exp loss from it. You can get more lives, so if you're evenly slightly careful, you'll avoid permadeath. But, well...let's just say, perminent losses to your character's skills or, worse, attributes can happen at higher levels, and various guilds (Erisian liberation front, I'm looking at you) can be extremely, extremely difficult to play. That said, there are easy options, and I may just not be playing the game right, for what that's worth.
Hope I helped.