Me too. I am hoping that the new Captain Marvel movie will open an unknown part of the MCU. I don't want the MCU to introduce the FF as "new" heroes. Maybe they existed in the nineties but have been trapped in the Negative Zone for the last twenty? I would love to see the FF and the X-Men retconned in some way that they have been around for many years but have not been publicly known.
I think it could work for them generally speaking, but there does come a point when the MCU has gone to this well too many times. Cap frozen in the arctic, Bucky and Zola experimented on by Hydra, Star-Lord abducted by aliens, Ant-man's secret cold war career, Captain Marvel off somewhere in space. Is it really worth it to add the negative zone to that list?
But if so, then at least give them something different. Not the 90s. The 60s are a good aesthetic for the F4's jetson-esque brand of sci-fi. Or forget the 'they're from the past' shtick and make them the MCU's first parallel universe heroes.