My ultimate theory for the endpoint is timey Whimey Dr Strange outlines either Stark or Cap needs to die for success... Stark petulantly sends Steve back in time to ‘save’ him and old man Steve shows up to make the sacrifice.
Therefore Steve dies, but can appear forever more in cameos as needed pre-Infinity War. Especially with Feige essentially outlining the MCU will forever hinge on before and after the events of Infinity war.
The destruction of the Infinity Gauntlet, or whatever they do to it in the end, will give them a McGuffin to start bleeding the universes together. I.e. I think the X-men will occupy their own parallel universe but can start to bleed between. Perhaps the event creates the Fantastic Four when Captain Marvel is also subsequently blasted back to the 80s, etc.
Or Maybe even one character gets blasted between universes to the new X-men occupied one, Wanda/Scarlet Witch?
But some other minor characters will truly need to bite the dust. I’d actually be fine with a Guardian character or two. The Thanos arch is really theirs first and foremost.