This... I hadn't really considered it till now, but the rest of the world really wouldn't just let the fantastic four or foundation or whatever just fade into the sunset. They have too many friends and family around to just let it go... Also there have been a LOT of substitute members in the past, so I could see someone picking up the torch... so to speak.
Spider-man and She-hulk were the first two I thought of.
Black Panther I would not use for anything... I really don't like the character, and I like him even less on teams. He's been too 'Mary Sue' for too long to really make it seem like anything other than T'challa and his subjects.
Dr. Doom??? Honestly, he's the same issue with Black Panther... except I'm not sure what he's like in his 'iron man/good guy' mode... so that could be somewhat interesting. Especially with the way Doom being involved with why there is no Reed or Sue around anymore... could be interesting, but I don't see it working too well.
I'd love some super genius level character to instigate the space travel/dimension hopping/exploration concept that FF was famous for... but can't really nail one down outside of Doom... Maybe Hank Pym?? But not sure he had any real connecton to the FF.