The big mechanic that many people have been suggesting other than ideology (which I know
@acluewithout has been pulling for) is an economic victory/corporations added to the game. That's somewhere they could also try to pull from a la Civ IV, but they are running out of mechanics ideas, I agree.
Nobody notices
corporations kind of clash with ideology system (and religion system as it is)?
1) One of three ideologies, communism, would be obviously not using them, not interact with them and just want to completely destroy ideologies allowing them (I have no idea if vague autocracy flavour should use corporstions or not). So you spend a lot of effort adding na entire large mechanic to the game, which is explicitly contrary to the nature of roughly 1/3 of endgame routes (and civilizations in the world). Not only communism has no other equivalent fun mechanic, not only it by its very nature is unbreakable wall to corporstions and money of 'capitalist' civs. But its existence makes also any economic victory besides vague-fitting-all-ideologies-idiotic-get-x-gold limited to, again, only 1-2 of 3 endgame ideologies.
2) Compared to this issue there is also a smaller one: corporations from civ4 are very similar to religions from civ5 and civ6.
- Limited amount of Cs and race towards them
- Holy city/headquarters city
- Spreads to cities and competes with other religions/Cs
- Gives you bonuses to your own city and benefits from converting others (and yes, religion is ofc often money making)
I think
corporations and economic victory are impossible thanks to civ5 mandatory ideology system (in civ5 it would be even worse in practice because AI loved communism and like half the world took it in my games

)
and very heavy overlap with religion system and religion spam victory.
Environmentalism? Completely covered by GS.
Civil wars and revolutions? Some sort of it covered by ideologies. Split states are impossible for civ because each nation needs to have animated leader...
Big future era? Covered by GS.
Missing civs? Truly obligatory ones could be covered by 2-3 DLCs.
Scenarios? Devs don't care about them in civ6 (shame, I actually really like those in civ5)
Migration? I won't comment on this as I think it was stupid civ5 mod to begin with (any self respecting non democracy would just ban it, it is nonsensical before industrial era, the mod made game ridiculously unbalanced etc)
Any revolutionary mechanics? Too late for them, too many other systems they'd involve, the game got too complicated to rework fundamentals.
Diseases? I think if they wanted they'd introduce them in GS, not just scenario. Besides, civ6 devs are afraid to introduce any mechanic with only negative side (they had to add nonsensical bonus yields to every environmental disaster).
World wars? A matter of single new diplomatic option and under ideology system. Seeing insane way Congress works now (apparently in civ5 it was too realistic and dangerous) I don't even believe it would be introduced properly.