tbh I instead love IOs, they're unique enough to not feel stale (since event scripted ones can have lots of flavour) while being very modular (thus making implementing said flavour easy)
I imagine stuff like trade leagues, confederations, imperial systems etc etc
for example, let's take Spain :
Spain once hits GP status and has at least a colonial nation as a subject unlocks the "Spanish Empire" IO, in which all colonial nations under Spanish rule and Spanish IOs become part of. This IO unlocks its nifty actions etc etc. Gameplaywise it makes it much easier to visualise what the Spanish empire is, and it gives the player a nice tab to see that (instead of a cluttered and unreadable diplo tab)
same stuff you could make for England, France, the Ottomans, Majahapit, an Italian confederation, the united provinces of the Netherlands, so on so forth
it has honestly just so much potential, and I love it