NO reason!

I think I need to step in here in defence of McMahon. Whilst I agree that Doom wan't evil in this film it wouldn't have worked if he was.
This story centered around his transformation into Doom which is a gradual process. Up to the point of his transformation he wasn't an evil person was he? He was a mega-successful business man, arrogant with an ego, possibly corupt but hardly what you would call evil and nowhere close to what Dr Doom is.

I think that with these comic book or Superhero movies characterisation is always a problem because the origin gets in the way. If you look at the 60's Batman series and I know people arew going to say the show was camp, a kid's show etc and you are right, but take a few seconds to really think about it.
How many of those had origins storylines? The answer I think is One. That involved King Tut gettig a head injury.
The first time you see The Joker, he is in his make up, costume and has a history as a villain. When Commisioner Gordon says to Batman, "It's The Joker!" Batman doesn't say "Who?" which straight away tells the viewer they have a past and have met before. the same with The Riddler, The Penguin, Catwoman etc.

By doing origin stories it does show how they got their powers or what their motives are but it also impacts the characterisation because it means they are never how they are in the comic.
IMO it was a mistake using Dr Doom in this film because justice was never going to be done to him no matter who played him. He came across as a character added to the film to please the fans. This was a film about how the four got their powers. Once they had them where do you go from there? You can't stop the film there, but if you bring in an antagonist for them to battle it needs to be someone powerful enough to stand up to them but also not someone who conveniently just happens to have superpowers as well without going into space to get them.

I don't think this franchise needs a reboot because if they do the same thing will happen. The film will have to focus on how the four got their powers. Whilst also finding a way to do an origin for their antagonist which would mean he/she isn't like their comic counterpart again. What's the point?
Pick up from where the second film left off using the same actors and move on with the franchise.