Doesn't matter. I describe how I see a possible MCU reboot, not what kind if reboot that IS GOING to happen.
With that said, I came up with the idea how to used to FC mutants instead of the original cast.
After the event of Dark Phoenix during the 90s, the mutants went into hiding. It was for the best because they got too much bad attention after one of the good guys turned bad. That's one of the explanations they aren't mentioned in neither Raimi's Spider-Man or Hulk, because they haven't been seen for several years. And could also explain partly why the military was so eager to capture/control Hulk. They know how much devastation superpowered beings can make.
And that's why Spider-Man is viewed as a possible threat at first, in both incarnations of the character (more so in ASM, but more about that later).
First Hulk happened and it didn't went unnoticed. Then Doc Ock wrecked havoc in New York.
So after the major event with Sandman, Venom and Goblin (2007) - Spidey was almost killed in a brutal manner - the mutants decided to finally come back into the world after being gone for about a little more a decade. They understand that people need protection from all these weirdly powered criminals. Like Peter said "were do all these guys come from?"
It could sort of make sense that they could have been hiding and then come back.
But not only because of past supervillains, but a new even more dangerous threat is on the rise. Xavier have noticed something with his mind-reading and/or tech
I think Raimi's Spider-Man would be best to go further with. It's close enough in time for Hulk and for both incarnations of X-men. He could actually meet the original X-Men, it's not impossible to blend the tones of the two frachises. Although Spidey being lighter at times, the fights could be violent.
After the loss of Harry, Peter might consider leaving Spider-Man behind him.
But something happens and he must again save the city. Does it have something to do with mutants? Or his Hulk back in town? Both original X-Men and FC can suit this idea.
I also see it possible for original X-Men also hiding for some years after the devastating events in Last Stand. They caused too much trouble fighting among themselves, and there were this massacre at Alcatraz, killing many humans in the process.
They then come back, and meet ASM version of Spidey in 2014 (who's torn and in grief after the death of Gwen). X-Men and ASM can work good together because they both present somewhat darker worlds. Maybe Peter has already teamed up with Daredevil by then?
What's the cause of the mutants' return? Has Ghost Rider arrvied and looks to be an evil demon? Or is it about Blade and the hidden world of vampires?
But in this scenario, we must perhaps leave out Ang Lee's Hulk since too much time has passed (11 years).
Now have have presented different suggestions here in a couple of posts.
Unfortunately none will take place at the same year of release, but years ago. That's the biggest obstacle when returning to older storylines for a MCU reboot.
All of them could even be soft reboots, meaning we could take the good parts and leave out the bad stuff. And have hints that the characters are the same from those storylines.
A different set of actors will even make it seem like it's something new, even it's not completely new.
Raimi's Spider-Man/Hulk/original X-Men (will be set in 2007-08)
Raimi's Spider-Man/Hulk/FC X-Men (-II-)
Amazing Spider-Man/original X-Men (2014).
Dislike it if you want to. But there's atleast stories to continue from in all three options.
No need to rebuild an MCU reboot from scratch.