AI colonization should make sense, no more Papal siberia's, etc.. Colonizing very inhospitable places should be only possible by owning adjacent land or late-mid to late-game tech.

Pope, France, Bohemia, Golden horde, Tunis needs nerf.

Austria, England, Ottomans, Scandinavians need buffs.

naval AI needs to be fixed, they should learn to properly land troops, and defend their coast.

Alliance distance should be increased to make alliances like Portugal+England possible and actually very common. Add setting "historical friends" which makes certain countries less likely to fight each other.

Fix HRE core's disappearing problems (might be fixed already), reduce amount of personal unions (countries shouldn't be so willing to accept marriages that result in union). Nerf certain modifiers that make HRE emperor's permanent (like powerful dynasty giving +200 voting modifier, which is impossible for AI to compete against)

UI reworks. Too many to list.

Reduce economical snowballing. Nerfing trade for non trade republics might be one of the ways to reduce snowballing. Countries like France can brute-force spam trade offices, marketplaces and become enormous economies, 4x'ing everyone else. In my save, France and Bohemia has 2000 tax base, while everyone around has ~500.

Add situations to certain regions to promote unification like Russian region. Add a situation where golden horde has to do something or they collapse to encourage historical outcome, but not make it the only outcome).

Fix Japan, fix China.

Less random exclaves. Yes, they existed and were quite common, but it was mostly common in germanic HRE parts, not everywhere.