I definitely think that the Medieval & Renaissance Tech/Civic trees need a bit of fleshing out......as they seem the Ages that can be traversed the most quickly. I think they could also look to further revamp the religious part of the game (& the religious victory as well), in part by bringing back & fleshing out the old State Religion concept from Civ4. More upsides & downsides for religion, as well as the means to alter the rate at which various religions spread, would also be cool. On a more aesthetic note, I really want to see them add a more diverse terrain pallet, especially for different land-masses/different latitudes. In the same vein, I'd like to see certain bonus & luxury resources be unique to and/or more common on specific land-masses/latitudes. Lastly, I'd like to see them revamp the Bonus & Luxury resource mechanic, the way they did with Strategic Resources in Gathering Storm. Concepts like Food Diversity, varying quantities of Bonus Resources providing varying, empire wide benefits, varying quantities of luxury resources providing varying, empire wide benefits......the ability to trade Bonus Resources. That kind of thing.