The biggest obstacle to creating a challenging Domination / Conquest victory objective is that once you've conquered one civ, you've conquered them all. The rest of the game is just cleaning up. Having to organize a cross-ocean invasion may be tactically interesting enough to be enjoyable, but the outcome is not in doubt. The only ways I see to put the outcome in doubt is either:
1. Create enough penalties associated with conquering civ one that conquering civ two, civ three, etc. becomes harder, rather than easier. This has mostly proven true in the real world, but mechanics to reflect this in a 4x game would likely be unpopular, at least amongst a large portion of the game audience. It could create a real domination challenge, though, that would be welcome by some, if each new war became harder and harder to win.
2. Create an AI that (a) ruthlessly pursues aggression so that during the end game the player is facing a unified second half of the globe that's as powerful as they are, and (b) is tactically smart enough (or given enough AI-only bonuses) to potentially beat the player at the complex late game warfare. See Leyrann's quote of stealth_risk for why this would likely be unpopular and see any conversation about Civ 6 AI for why it's also almost certainly technically unfeasible without AI "cheating" (which itself would give birth to such a black hole of discontent on Steam/Reddit that all positive Civ 7 vides would be sucked into it, disappearing forever beyond the event horizon).
3. Load the AI civs up with enough of a starting bonus on Deity that surviving long enough to conquer civ one is enough of a challenge that players are content to then enjoy their laurels as they mop up the rest of the map. This is likely where we'll land.
