My intention is just to give some thoughts, or if a modder want to have a (non-modding) partner.

- exotic wares losing its values (inflation?),
- rising cost on buildings (depending on location, development, supplies, infrastructure, inflation, ...?), maybe even improvement?
- give rulers more "autonomy", hence more "stupid monarchs" set objectives (if not reached, he/she gets more frustrated) and may declare war (even though you know that is detrimental for the future), whereas republics doesn't have that high unpredictability (depending on corruption), but more difficult management of internal stability
(- or monarchs and dukes rather spending too much on courts than on development)
- other thing, in my opinion, that may give the game more difficulty (as in: what happens to me happens also to my rivals!): no more exact data, especially on army and navy. The player and the AI has to send spies to get informations, and even then it is only an approximation! I think, that way, wars CAN become unpredictable; meaning: you lose some, you win some (as do my rivals!)
- complacency: often defeats and paranoia initiated a renewed thinking/approach
- give more weight to culture? As new cultures can grow from within. Assimilation is very costly and takes generations ...

It is weird ... I would like to lose some wars with the knowledge(!) that time comes to my rivals as well! But once I have grown to a size, I get complacent and I am not that keen to conquer and conquer; though(!), what if my rivals unpredictably declare war and peace demands are not provinces anymore but states, maybe I get a bit more panicky ...

Anbennar's Rianvisa (Venail/Aelnar) was one of the best crises I ever had, keeping me on edge for a long time ...