If your really honest and creative enough you will realise that all X-MEN universe properties could be translated onto the silver screen...
The idea of the x-men movies is to retell the classic stories from the x-men universe blending it with a realistic tone... Mutation is realistic but not to the extent of the comic books. So in order for them to work they need to take the normal... the realistic and add that one element... Mutation... if we were to go into any other unrealistic areas your faced with it moving away from Science Fiction to unrealistic Fantasy... and thats what we dont want to do... But let me explain alot of the troubling areas that can easily be adapted to the big screen.
SPACE
> What you got to remember is if there are mutants that can control magnatisum, then i'm guessing there are mutants that can breath in space... or just dont need oxygen... so Mutants in space could be plausable... But you couldnt base too much of a story around it. There is no real social issue that would need for such space exploration... But picture this... What if NASA used Mutants to work on a space station to track down other mutants... or even still just to do Experiments in space... with mutants who dont need to breath or need protection from radiation... As long as they keep it to a minimum, space could play a tiny part in the x-men movie... but not along the lines of the x-jet flying off into space... no i dont think so...
ALIENS
> I've been giving this alot of thought and well we cant explain spacepeople it would be too unreleasitic for the overal story. The best alien plot would be the Phalanx as its somewhat explainable but even then its bording along the lines of spiderman 3 with Venom... But the way i see it lets take out all the meaning we know aliens to be and look at the other meaning... Alien, foriegn, isolation... although we cant explain Aliens from space but we could use the social issue of been alienated...There mutants!
ROBOTS
> If it was upto me, id leave them out completely... im not a hugh fan of the sentinals. There translation to big screen would be going just that little bit too far. But alot of people want to see sentinals and well they can be explained... The goverment although more excepting by the end of x-men 3, they are defenseless against a mutant attack and i guess it would be fair to say that they should be able to defend themselves if necessary. The way i see sentinals is the way i see How Terminators were created... First you started off with little helicopter things, small personal tanks... look at salvation. Terminator Dog-Bikes, River snakes... starting off with small things before scaling and each time modeling down to we get the familiar Terminator we all know and dare say...Love. Sentinals should be treated the same way Trask should have Sentinal Mark 1, Mark 2, Mark 3... used for survaliance, data gathering... then Mark 4, issued with mutant supressents it gets darker and darker... I had an idea. I was thinking about The Reevers and i thought we could adapt them... a group of people somewhat submerged with technology... i mean not literally like in the comics books. But picture this... The Goverment creates the Sentinals as mutant tracking device an idea produced by Henry McCoy and Bolivar Trask. In a bid to help keep mutant threat levels low sentinals are produced to monitor and track any high risk mutants... But then we have The Reever's... a group of hackers, technology specialists and very Anti-Mutant believe that humans have no defense against them and they feel cheated by the Goverment so the Reever's adapt the sentinals Mark 4 to seek out and kill mutants.
MAGIC
> Juggernaut wasnt explained as magic but just that of mutation... if think any Magic bound character from the marvel universe it can easly be explained as mutation...Unless your making a Dr.Strange movie but thats a whole different genre.
Now i am going go go through some of the most extreme characters and explain easily how they could appear...
MOJO
> Although he is an alien, what he stands for is quiet unique and a concept they have yet to show in the x-men movies. Picture this... we dont make a X-Men sequel but a spin off with characters that appear in the x-men comics but not the movies... What is Mojo was a business man, showman, entertainer in a backwards secluded city that has an arena... that arena houses sports like monster trucks, car racing, mutant hunting... wait what was the last one. Yeah some backwards city hires entertainer Alison Blaire to perform at there next Mutant Hunt... although she is unaware she soon finds out the true cause upon her concert. As her powers are revealed she is saved by LONGSHOT a mutant who has been hunted for several months now and is able to survive due to his natural good luck.... The group of mutants must escape this town fighting some of the darker more sinister characters in the caterlogue of x-men world. Now i could see Longshot being a good Horror based Marvel movie...