I guess the most easy choice is let them have generic trees for common things like a industry buildup and always just add a few decisions for every minor.
(Ireland--->getting northern Ireland for example) More is better, sure, but i would let handle the mods.
Normally i would say thats a developers job, but honestly its different with HoI 4 and most likely stelleris too. Both seem to be constructed to be easy modable,
and the modding community in this forum is one of the finest i have ever seen in the www. So i really think paradox schould focus on game mechanics and content,
while the modders
polishing the diamond.
In that thing i just want to add:
Thank too all of you people who worked on Mods for HoI 3 and other games!