Adding new civs and hopefully make them stay unique. I vote for Norway, Finland and Iceland. C'mon gotta squeese those last (largest) nordic countries into this great game. Even though there are great civ mods for finland and norway, Im curious on how Firaxis would have made them, maybe credited the modders with swag and booty for borrowing the features from their mods.
Also, yeah theres mods for it to but would be nice to see added pagan/ethnic/ pre-christian (muslim etc) religions too, which fits for each existing and coming civs.
Last, working out the quirks and kinks and improving the game overall of course.