Changing the UI is a separate code template, I would think.
Gotcha. Hopefully, it should still be possible. I know that EU4 modders were able to squeeze a great deal more flexibility out of certain windows, so it's likely that it'll be possible in Stellaris as well.
I know that adding new ethos/government types would necessitate a fair amount of new/modified events/edicts/policies in order to bring them up to par, as well as figuring out how it affects POPs.
Anyways, other things that I'd like to change includes getting rid of the 5th "neutral column" and instead having individualist and collectivist government types. With meritocratic/egalitarian governments, and the Anarchic row, that would bring us up to 32 government types. I think that this would be an improvement over the current 15, although it would require unique edicts/policies/events as well as new sets of bonuses/maluses in order to make them all feel unique. While the final product may differ, the previews seem to indicate that differences between different types of governments (as opposed to differences between monarchies/oligarchies/democracies) will be relatively minor in the base version, and I feel that this is relatively unsatisfying.
So, general ideas, with some assumptions on the possible types of bonuses/maluses:
Individualist
Monarchy: Star Kingdom (Feudal Empire)
-Pros: Levy edict. Greater expansion potential.
-Cons: Planetary governors have more "autonomy."
Oligarchy: Plutocracy (Merchant Republic)
-Pros: Greater wealth creation. More trade potential?
-Cons: More war weariness. Greater apprehension to taxation.
Democracy: Representative Democracy
-Pros/Cons: Probably equivalent to vanilla Indirect Democracy.
Anarchy: Egoist Society
-Pros: Increased leadership experience. Increased tech progress.
-Cons: Reduced stability. Increased colonial "autonomy."
Collectivist
Monarchy: Despotic Empire
-Pros/Cons: Probably equivalent to vanilla Despotic Empire.
Oligarchy: Bureaucratic Despotism
-Pros: Increased stability. Reduced war weariness.
-Cons: Reduced societal tech progression. More costly expansion.
Democracy: Consensus Democracy
-Pros: Reduced unrest.
-Cons: Longer policy cooldown. Increased chance of democratic crisis
Anarchy: Hivemind
-Pros: No unrest. High slavery tolerance. Unique gameplay.
-Cons: Diplomatic penalties. Tech penalties. Unique gameplay.
Meritocratic
Monarchy: Libertarian Monarchy
-Pros: Increased wealth creation.
-Cons: Reduced ability to issue policies/edicts
Oligarchy: Corporate State
-Pros: Increased wealth creation and trade. Unique gameplay.
-Cons: Decreased stability. Unique gameplay.
Democracy: Citizens' Democracy
-Pros: Increased leadership experience. Reduced ethos shift rate.
-Cons: Reduced ability to integrate other species. Reduced ethos shift rate.
Anarchy: Market Society
-Pros: High trade potential. Easier species integration.
-Cons: Higher unrest. Increased colonial "autonomy."
Egalitarian
Monarchy: Revolutionary Empire
-Pros: Decreased war weariness.
-Cons: Decreased stability.
Oligarchy: Vanguard State
-Pros: Cheaper policies/edicts.
-Cons: Reduced wealth creation.
Democracy: Council Democracy
-Pros/Cons: Compromise between Direct and Representative Democracy.
Anarchy: Syndicalist Society
-Pros: Reduced unrest. Bonus to production.
-Cons: Reduced wealth creation. Increased colonial "autonomy."
Then of course, there'd be tinkering and adding 4th row governments for the other ethos.