Everyone has questions about the future of the MCU after Endgame Im calling it now, that Disney will form the MCU with the same idea done with the Star Wars franchise.
>Phase 1-3: Barrage of large movies ending in concentrated melee battle >Phase 4-5: Slow character movies with little to no team ups with prequels, small overlaps,etc... >Phase 6-?: Another barrage of large movies built up on nostalgia and Phase 3 plot holes
Whatever comes after Endgame will likely be a slow buildup to an Avengers: Secret Invasion movie. BP is a King and therefore a potential target for infiltrators or there could be more minor characters that have been replaced secretly. Wong, Shuri, MJ etc. could all turn out to be Skrulls and then the New Avengers unite with Captain Marvel to stop the invasion/Super Skrulls or whatever.
They might even cross it over into the streaming shows since Feige said they'd be relevant. Vision and Wanda have Skrull refugee neighbors, Bucky and Falcon fight terrorists that are secretly Skrull etc. Things like Guardians 3, Eternals, Strange 2 and Loki might be building to some other kind of cosmic threat further down the line though, but I think the Skrull stuff will come first, the "trust no one" marketing campaign is too juicy too ignore. And they can even tease everyone by making them think Tony is alive again only to realize he's just a Skrull.
But we all know X-Men and F4 are a long way off from even being hinted at according to Feige, so I won't be expecting them anytime soon.