The two major changes I would make:

1.) Video card compatibility. (Yes, it's an obvious one, but it needs to be said anyway).

2.) Improve/fix the AI, especially on the campaign side of the game: No more allies backstabbing you for little or no reason. No more 1-province factions attacking 30-province superpowers. No more constantly retreating their way to death across the campaign map as you invade -- for gods' sake computer, at least defend your capital! No more randomly attacking my ships (but making no other aggressive moves). Better handling of fleets and trade. A more realistic assessment of its chances of victory when considering war with another faction (sure, the AI may take that first province, but can it withstand a counterattack?).


There are certainly other things like unit balance and build requirements that could be altered, but the first two are the biggies for me.