What I wish they'd add is online multiplayer with more than 2 players. Two of my friends and I are into Total War, and the online multiplayer saves time over co-op which really adds up, but we can't have a game with all three of us in it since AFAIK even as of Rome II they don't allow 3 players online. Maybe Attila does? I'm not really interested in Warhammer as I never played those games.
Medieval II is still a favorite, the timeframe is great, and the game's held up well with time all said. For the diplomacy/factions and non-splintered Rome alone I don't think I could go back to Rome I after having played Rome II, but Medieval II is still a good time. I'd be curious to see what they'd put into a Medieval III - and it wouldn't surprise me if that were the next game in the series.
Edit: Nope, looks like Attila doesn't have it. This is probably the one feature that could make the next historical Total War game an first-patch buy for my friends and I (because let's be honest, Total War always needs a patch or two), rather than a "this will have a 75% sale in a couple years and we still have 12 factions we haven't played in Rome II" buy.