tl;dr: Probably not.
Is Stan Lee playing The Watcher in all his Marvel cameos?
Almost certainly not. And if he is, they will never reveal this.
Brief historical tangent (for the, like, three people who don't know this already): from the 1970s to the early 2000s Marvel, unwilling to self-finance movies based on their properties, parcelled out the film rights to a whole bunch of their characters to other companies. The full legal history of this is complicated, but the upshot is that Marvel still doesn't have the legal right to bring some of their characters into the Marvel Cinematic Universe. At time of writing, there are basically three groups:
- Sony Pictures, who own Spider-man
- 20th Century Fox, who own the X-Men, the Fantastic Four, and Deadpool
- Marvel Studios, who own pretty much everything else1
The Internet is unclear about just who owns the film rights to Uatu (the two most likely guesses are Fox, through the Fantastic Four franchise, and Marvel itself; though Marvel appear to at least have the right to use the Watchers generally), but barring a special arrangement they can only be owned by one studio. So barring some unprecedented co-operation between these three competing companies, Stan-as-Uatu will fly in one, at most two, of these disparate universes.
Okay, but what about just the MCU movies?
Close, but no cigar. Lee's cameo in the 2017 Guardians of the Galaxy, Vol. 2 suggests that at least some of Lee's cameo appearances have been the same character, but it's made quite clear that he isn't playing a Watcher personally; as Rogue Jedi remarks in an answer to a related question, he's credited as "Watcher Informant."
As reported by ScreenRant, Marvel Studios President Kevin Feige has made statements that may imply that all of Lee's cameo appearances (or, at least, the MCU ones) are indeed the same character (emphasis mine):
[W]e put it back in towards the end of the process where he references that time he was a Federal Express agent – we thought it would be fun to put that in there because that really says, so wait a minute, he's this same character who's popped up in all these films.
For my part, I don't know that I'd go so far as to say that this is explicit confirmation that they're all the same guy (he's had an unbelievably storied career if it is), but it does seem clear that at least some of them are the same. However, rather than the character popping in deliberately, it seems as though his frequent appearances are just happy accidents.
Recent revelations notwithstanding, the exact scenario described in the question is nigh-impossible:
[I]f Disney ever gets the rights to characters like Spider-Man or the Fantastic Four or X-Men, they could introduce the Watcher, (not necessarily played by Lee—maybe in his real form) and we can get a montage of scenes of Lee in previous films, observing the Super-Heroes of Earth (and space).
This isn't how rights work. Even if (for example) Sony relents and reverts the Spider-man rights to Marvel, Marvel will only have the ability to make new movies with the Spider-man characters; they won't own the rights to the Raimi Spider-man movies, or to the Amazing Spider-man franchise. Attempting to use clips from those movies will require Marvel to come to an agreement with all of the rightsholders, which is a logistical nightmare as well as a financial one. It would be different if Sony Pictures was purchased by Disney, the same way Marvel was, but this seems unlikely in the foreseeable future.
The only sane way this could happen is after superhero movies become unprofitable, but by that point, even if Stan Lee is still alive, would anyone even care?
There's a slight complication introduced by Captain America: Civil War, which marked the beginning of a co-operation between Sony and Marvel that brings Spidey into the MCU, despite his film rights still being owned by Sony.
This development does make the possibility of a Lee-as-Uatu multi-universe crossover more of a realistic possibility, but I still find it unlikely for all the reasons mentioned above, especially the fact that only the Tom Holland version. Even if the possibility exists, there's still no evidence to suggest that it may happen.
1 There are some complications, though they're slowly un-complicating; the rights to Namor are a bit up in the air right now, and there's famously a special agreement between Fox and Marvel that allows both studios to use Quicksilver and Scarlet Witch under special conditions.