Welcome to the forums!
It depends on what you expect of course.
At this time, there are no real "complete overhaul" mods, like Magna Mundi for EU3.
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Imperium 2.0, by
Descartes, comes closest in this regard. It just might be a work of genius, but it is still in beta (and probably will be for a while). It adds a lot of new graphical touches, and heaps of options -- but if you are a purist this mod might be a bridge too far for you.
If you want to stay closer to the orginal game, I recommend:
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Lofman's Little Mod, by
Lofman (of course), mostly adds new features related to warfare.
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Epigoni Mod, by
Hardradi, which actually is a combination of his previous work. Adds river trade, wonders of the world, a lot of nations and nation decisons. The focus here lies mainly on the eastern mediterranean and the late Diadochi.
If you want an expanded map, try:
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RIMP or the new
Magna Terra (haven't tried these myself though).
There are probably some hidden gems out there too. The community for this game is not the size of EU3's, but it's a friendly and helpful lot.
Have fun!