It wouldn't at all surprise me to see some sort of connecting dual credit thing after End Game and Dark Phoenix. Given all the reality warping potential of both movies it would be an easy way to introduce the X-men into the MCU. And with the reality warping they could keep the parts of the X-verse they want to keep because its working (Deadpool) and reconfigure the other parts they have to change due to retiring actors (Wolverine / Hugh Jackman) or the X-men being stuck in the past. The way you describe it would work perfect because the implication is there without having to worry about casting.
That said I fully expect one of the end credit scenes to be a tear jerker scene in some sort of Afterlife where now deceased Steve Rogers, who I expect to die in the Endgame, finally gets that dance with Peggy.