Making Annihilus a major villain after Thanos would be welcome. He has a very Alien, demonic look even. I think you might be right, time period and all, though there is great symbolic meaning to taking them back to November '61, the date of their first comic book.
Still without a total reboot of some sort (which I'm not necessarily against), inserting them in a retro way (Pym is a contemporary of Reed's, before they were lost in space or trapped in suspension by the Inhumans on the Moon, my idea) would be the way to go.
I have no idea how to retro-insert the X-Men under this idea. But if they were acquired by Disney why not a roll of the dice says somehow their universe/reality merges with our (MCU) one? Since this is aftermath of A4, 2020, etc, than it could be the innovative things Marvel claims to be happening. And thus in - story it would be handled more like that (Infinity's Reality Stone), than how they would retro insert the Fantastic Four.
They just wouldn't want both of their insertions to be plot-based the same way.