So lets just get this out the way, X-Men (or whatever team name they decide for the MCU) are coming. It might even be hinted at as part of Phase 5/6. If I was to guess the next Saga after this Multiverse Saga wraps up will be the Mutant Saga, so that'll be Phase 7,8, and 9. And that will hopefully be capped off with an Avengers vs X-Men movie at the end of Phase 9. And if that comes true that will be the MCU movie that could seriously rival and surpass Endgame if Kang Dynasty and Secret Wars don't.
Also as
@MisterPenguin stated, every character in Marvel has been on one of the Avengers teams at some point. So yeah they can actually just slap "Avengers" on whoever, because that's just how the stories roll. For example we're going to start seeing a lot more anti-heros coming into the MCU, and they are going to be added to the Avengers team to be able to battle Kang.
Also I don't think you give the source comics the respect they deserve. They are a lot more mainstream now then you realize, and yes the MCU is partially responsible for this. Its no longer as niche as you believe it is. This is not the 70s and 80s, its no longer isolated to the readers living in their parents basement.