BNW is an awesome expansion that's brought oceans of depth to the Cultural and Diplomatic Victory games. The down side of this is that in contrast the Science and Domination conditions now seem pretty lack-lustre affairs. The attainment of the Science Victory has always felt a little abrupt to me, especially when a rival Civ makes it to Alpha Centauri first; causing an 8 hour game to end without any kind of fanfare. The Domination game too has suffered from a lack of depth in Civ5 with the A.I sometimes struggling to organise attacks in a satisfying manner, and now with addition of the second expansion the military road to victory seems like an especially flimsy one in comparison to its Cultural and Diplomatic alternatives. With this in mind I'd like to see a revision of the two aforemented victory conditions in a third and final expansion to Civ5 that presents itself as a naturally complementary counterpart to BNW by creating a deeper focus on the Science and Domination endgames.
For the Science game I'd like to see emerging real world technologies realised in the Future Era (plasma reactors, hydrogen fuel cells, spin-off nano tech, genetics and cybernetics for example) leading to a Science endgame that concentrates on setting up Moon and Mars bases as a staging post to mine the asteroid belt for Iridium; the essential resource required power an interstellar flight system. A system with limited colonization slots available to build off-world bases on a simple map of the solar system would be pretty cool; adding an element that parallels that of the World Congress for the Science Victory lover.
As for the military game, why not introduce the ability to make mini-stacks of five or six units when Great Generals appear; straddling the systems employed by the earlier Civ games and Civ5 vanilla with an interesting and workable compromise that avoids the dreaded stacks of death found in the latter iterations of the game and the chaotic problems created by unit saturation in this one. Ideally I'd like to see Great Generals and Admirals able to stack a few units into a campaign army or armada with incremental bonuses added with each new unit included. Perhaps the right combination of defensive/offensive/ranged/cavalry/siege and scout units could confer special attacks unique to each Civ too! Surely the A.I could handle attacks more shrewdly and with more economy with this system in place. Of course a revised Future Era tech system could also go hand in hand with a revised military game by offering techs that made attacking rival Civs more immediate and dynamic as the endgame approached.
To cut down on the monotony of constantly shuffling units and micro-managing the masses of cities that come en route to a Domination Victory I'd like to see a new "capitulation" mechanic added where if a number of enemy cities were pupetted along with the Capital the whole target Civ would capitulate and become a satilite state that would self manage, following broad edicts that could be set via an "imperial" sub menu; creating yet another parallel to the World Congress and new space colonization mechanisms.
Perhaps a counterpoint to the Archeology system could be added too, by allowing Workers to create super-structures in key geographical locations; things like the tunnels through mountains and under narrow sea channels, building ocean joining canals and suspension bridges, and in the Future Era: Nanofibre skyhooks to create space elevators, atmospheric processors to boost growth and terraformers to enhance production.
Add to these new systems a revison and streamlining of the now slightly clunky three tiered trade sytem (trade routes, roads, agreements) into something a little more coherent. Add a little transparency to the esoteric Diplomacy system by hinting at leaders' hidden motives (perhaps with the help of diplomats and leader bonuses.) Condense the various menus into something more easily digestable, for example by showing things like whether Wonders are available for construction in the Tech Tree and including more immediate and accessable Belief and Trade menus, prehaps through the addition of some nice new icons for the Religious and Ideological systems to illustrate a Civ's allegiences, resources and bonuses at a glance (oh, and they should really add some music when a Great Musicians appears, doh!)
A third expansion along these lines would provide a real uniformity to the various game elements in keeping with the groundbreaking new foundation that BNW has established for Civ5. If properly realised a third expansion certainly wouldn't feel like Firaxis were trying to stimulate a dried up teat, but rather like they were adding the final piece of the Civ5 jigsaw by finishing what they started with the launch of their second expansion.