Any intention to add something with peasant republics? I was thinking of something like: if date is after 1300 (not necessary, but all the medieval ones happened in the 1300s afaik) , enough peasant rebels controlling provinces within an area may cause an invasion which would install a republic in an area if successful. Preferably with plenty of special modifiers to make them less prone to mercantile domination and hated by nobles everywhere.
That would be a pretty cool idea. I don't know how the heck the coding for that would be like, although it would probably be something similar to the Hansa (at least how the republic pops up). I also don't know how really workable that would be - historically, if I am correct, many of the peasant republics got offed pretty quickly. I wouldn't want to have them arise ahistorically, that is, and I wouldn't want it to be a gimmicky thing. I think it would be cooler to implement rising republics in general, where certain regions meeting certain criteria could have a republic pop up, and different circumstances would lead to different styles of republics. So basically what I'm thinking is that a "peasant" republic might be good, but it'll basically still arise similarly (and perhaps function *somewhat* similarly) to other republics, only with different prerequsites. If that makes any sense? I've put rising republics on my to-do list, anyways. Probably in the far future, though.
Will one of the ancillaries be to have a dog?
Of course. Man's best friend and all. It was even the first ancillary I thought of, since I figured it was so obvious.
I'm not sure where this has been asked a long time ago, but what files are potential points of incompatibility between the overhaul mods and the core and event modules of your mod? For example, I want to run it along Ck2+ or Lux Invicta instead of Project Balance (PB is a nice mod, to be sure, but just not to my taste). Any specific files that have a history of incompatibility that I could take note of?
Lux Invicta has already more or less integrated the traits module of VIET (formerly the More Congenital Traits mod). The core and immersion modules are definitely incompatible, and while the events module is theoretically compatible (I think) it will probably cause weird things, though nothing game-breaking (namely events from Lux Invicta may not show while VIET ones do, or vice versa, I believe).
As for CK2+, core and immersion are once again definitely incompatible. The Traits module is *probably* compatible, though I don't know what new traits CK2+ adds (CK2+'s new traits aren't the issue per se, but rather if CK2+ adds the same traits - for instance I think CK2+ also adds its own Tall trait, so there might be weird situations where a person has two "tall" traits, one from CK2+ and one from VIET (though I don't know this for certain, as I've never really messed around with CK2+)). The events module, similarly to the case with Lux Invicta, should in theory be compatible, but again it may cause weird things (though I don't know for sure, as CK2+ definitely doesn't have flavor events along the lines of Lux Invicta).
I am planning to make a CK2+ compatible version of the Events module sometime in the near future, as several people have brought up CK2+ in this thread multiple times, and given that it's the most popular CK2 mod anyways I guess I might as well give you CK2+ fans something, even though I'm firmly in the Project Balance camp.

The other modules probably won't get CK2+ compatibility simply because CK2+ changes so many things it just can't fit with the way VIET if designed.