That movie set to come after Avengers: Infinity War is Inhumans. An obscure comic property that is currently being repositioned to be essential, they're being mooted to be the MCU's alternative to the X-Men, which makes them pretty important in whatever Feige has already planned for Phase 4 and beyond. So important that Marvel started seeding them four years in advance by introducing them as a key driving force behind Agents Of S.H.I.E.L.D. But, as we've already established, Agents Of S.H.I.E.L.D. isn't making any impact on the wider movie universe, meaning all this set up will be ignored, if not completely overwritten when we finally get to the standalone movie. In anticipation of that inevitability, the MCU needs to at least put in a token effort to establish these guys. How the Inhumans will be introduced, along with what elements will be adapted, hasn't been made clear. For all we know the majority of the heroes haven't been created yet and there'll be some Terrigen Mist genesis event in the future that transforms them, but, even so, some in-movie set-up of that event would really help get the characters across.
