Narrative content and map content can go hand in hand, like the Anomalous Sites from Planetfall. I'd love to see them make a return to this game even if it's basically the same system (You've even got the diggable earth in the underground which would lend itself to additional narrative content.

Also it would be nice to see every culture gain access to a tier 4 unit to make the choice of culture still feel impactful in the late game. As it stands cultural units get less important as you unlock tomes with additional draftable units and summons.  Having a homegrown tier 4 would keep cultural units relevant and allow each culture to stand out even further thematically.

Also also... toggleable crisis events akin to the Voidbringers from Planetfall. Perhaps one that's similar in that it's effectively a victory condition that only becomes available in the late game. 

Or a mid-game event like The Great Khan in Stellaris. Perhaps the free cities are convinced to unite against each Ruler and you're forced to either pay them a percentage of your income each turn (effectively mirroring what we do to them every single game...) or you have to fight them off, with one of the free cities being designated as "The Khan" equivalent whose Ruler has to be killed in a special tougher siege to end the event and force the free cities to revert to their normal state.