Alright, after all those joke answers will give an earnest one.
There will likely be one for decentralized countries, as wiz has hinted at that possibility. Other than that is hard to guess. For precedence, we can look at the two DLC for vic2. and the three major dlcs wiz helmed in stellaris.
these being:
A House Divided
Heart of Darkness
Utopia
Apocalypse
MegaCorp
We can also look at CK3's first major DLC Royal Court for possible company direction.
Now, It's most likely stuff introduced in Vic2 dlcs will be, atleast in some form, carried over into vic3, it's possible that they could retread the same topic. (another ACW or colonization focused dlc wouldn't be unreasonable)(the colonization one could even coincide with the introduction of playable decentralized countries)
It's quite possible the first DLC though, wlil be a Utopia/Apocalypse/Royal Court style rework/extension of major mechanics.
MegaCorp is in a way similar to that theoretical playable decentralized DLC and that will likely be the second DLC.
Ultimately its too early to really speculate effectively these kinda topics, but those be me two cents.