Just a few.

1) Repeatable techs. One simple one would be a tech that boosts your power output by a small amount. That way once you've researched everything you can continue to invest and improve beyond what would normally be possible. Like, even, say, a 1% increase in power can have a major impact when trying to keep a modern society going at full power.

2) A 'all wonders stealable' option for Sandbox mode. Pretty straight-forwards. You can have it disable achievements or something if need be.

3) A new tier of modern apartment building. Seems a bit weird to me that these towering skyscrapers house only 16 families. So add in a new 'small modern apartment building' which is what our current MAB is then include a 'Apartment Skyscraper' or something using the current MAB graphics but able to house, like, 32 families or something. I find it extremely annoying that housing is such a constant uphill battle simply because housing is so limited. I'm more than fine needing to wait till the modern era to get access to a solution, I just want one to exist.

4) More wonders in general would be nice. Some simple suggestions: Terracotta Army: (Colonial): Increases the health of armies by 25%. Notre Dame (WW): Serves as an island-wide Cathedral with a major boost to religious happiness. Big Ben (CW): Can enact any one social edict for no cost/penalty. The Hague (Modern): Serves as an extremely powerful Modern Arts exhibit.

5) New Era: The Future(?): Maybe this is a bit too far-thinking but adding in a fifth era based on the future could be interesting. You could have things like teleporter booths, flying cars, underwater housing, and so-forth.

6) Mega-projects: Can't be hurried and requires a large amount of resources to complete, but when done provides things like a super Nuclear Power Plant or a huge-ass apartment building with room for 500, or some other crazy stuff. This would work well with idea 5.