If Firaxis wants to truly improve the game, and not just cash in on the next sequel in the franchise, they need to focus on two things:
1. By far the most important thing is to make each age/era interesting. The early game is good - it's chock full of exploration, choices, planning, etc. It's a downhill slide from there, though, almost always leading to a 'meh' late game that is mostly an efficiency slog. The only challenge here is managing to see it through to completion. Late game should culminate in 20th century style world wars centered around ideology and world dominance, wars that are only won by effective use of combined arms warfare. The tools are there, I just hardly ever have to use them (unless I'm steamrolling an incompetent opponent in a domination run that I haven't managed to win yet for some reason). Late game should be the most exciting part of the game (what a thought!). I'm not saying every game has to exactly re-enact WWII, but that should be the general gravitational pull, instead of the yawn that currently occurs.
2. An AI that can play the game well, in a varied and interesting fashion, and be more than just a speed bump on the player's road to eventual victory. Challenge that is based on more than just stacking numerical bonuses for a mediocre AI. This would be a philosophical shift for the game, and so I realize this probably won't happen... but many players nowadays are far more interested in high stakes, rogue/rogue-lite type games, and I for one am ready to play a Civ game where my opponents can actually win, and not just potentially slow down or frustrate my plans.
I really feel that if Civ 7 is just "Civ 6 with more stuff," it'll be the first real failure of the franchise. Civ 6 has hit the limit for me on complexity and bloat - I want a leaner game that is more interesting because of the gameplay, not the mechanics.