Have you played the Fall of the Samurai expansion? I've enjoyed it so far, especially since I've missed shelling the daylight out of my enemies with artillery. The naval battles are vastly improved, and another addition is that you can use your entire army + reinforcements in a battle all at once, so there is potential for some truly epic battles.
I would also recommend Empire. It's a rather large divergence from Rome and Medieval 2 in that the core of your armies are no longer melee units backed up by archers and a few siege units. Instead, your armies will be made up of line infantry-type units supported by cavalry and artillery (cue 1812 Overture).
Also, there was more emphasis put on naval battles in that you can actually fight them.