Domi-Sub (Parent-child)? :3

If you haven't already, I'd add separate groups for age categories for sure.

Also maybe stuff like punishments (namely spanking)

If comfortable there's also a bit of overlap with omorashi and other bathroom-related fetishes

I dunno if these count as sub-communities per-se but they're definitely interests that overlap.

Crossover with pup community?
Bondage, either being dished out by the caretaker figure or by another babyfur playing an older figure
Undies?
wetting pants rather than diapers

Hmm... this is a tough one... I'm supposing this should be stuff that can be IRL? I don't really have any exp. with this stuff outside of the art I like to look at and fantasize about.

Bladder and bowel control, like being made to hold it or go on command (is that a thing?)

Is "sissification" the act of forcing a boy to dress like a baby girl?

Perhaps urethral insertion? Like using it to keep them from being able to pee/cum, possibly while locked in a chastity cage?

Oooh, how about being bathed? Having the bigger/older one washing your body, which could include enema play or inserting a plug afterwards.

And definitely Plusheys. Cubs who are into plushies either to the point of simply loving them or as far as Making love to them.

I see lots of cubs who are into various BDSM-related areas - like bondage, discipline, dominance, submission, control, pet, slave, humiliation, degradation, pleasure denial, entrapment, non-consensual, etc. None of those do anything for me, but they are extremely common sub-interests that I see in cub art.

I think wetting is very common. It's usually focused on diapers, but quite a few of us are into clothes wetting, bed wetting, omorashi, accidental, and accidentally on purpose wetting! This could overlap with diapered fursuits and diapered plushies. I guess messing is common too, but those who are into wetting aren't always into messing and vice versa.

I see cubs who focus mainly on things like playground and fun-time activities. The part of cub life that they are after is simply the gathering with other cubs and sharing in play, games, coloring, building things, and other fun, almost like an eternal recess. I'm definitely in that category.

Plushies and/or fursuits are also popular themes for cubs. These can also overlap with any other sub categories.

All of these things are very common themes for cubs.

Well this is perhaps me speaking but I know one sub group which I think would be interesting to see in cub/babyfurs is macro/micro. I myself have a macro babyfur and which has gathered a lot of interest. Seeing a regular babyfur is cute and adorable and one that is gigantic or tiny is just as cute and nice to see, no matter how big or small one is. So I don't know whether or not you are one for that sort of thing though. If you'd would like to see what I have in the way of macro babyfurs then feel free to take a look at my gallery.