Thinking about the city the obvious choice is Attilan but as already stated I doubt we will see that until the Inhumans film, and I would think this City will be found long before that.
The ruins in the blue area of the moon would fit, but given TV restraints and possible rights issues with the FF (first appeared in FF and The Watcher is with them for sure) they might revise that Kree/Cotari experiment concept so that all went down on Earth instead, perhaps in Antarctica, where they could merge it with the Savage Land too (and I think Mockingbird first appeared with Ka-Zar?)
The Eternals is another option. Similar concept as the Inhumans, and I think Thanos (and all Titanians) were ret-conned into being them? We have seen one dead Celestial and another in action in GOTG too so they might go with that.
Last one (I'd think this least likely) could be Atlantis. Not sure if they'd run with that though (having Atlanteans descendent of an alien race) plus afaik there's no plans for Namor in anything atm.
Whatever they go with I figure there must end up being an Inhuman connection there. My best guess is they might merge the Eternals with them (which has been done in the books. Thanos's son, 'Thane' is a hybrid of the 2). Next would be the blue area of the moon moved to Antarctica.