10. Nimrod
Marvel ComicsGiven that Sentinels have now been established in the X-Men universe, Nimrod would make sense as a future villain. He is a mutant-hunter from an alternative future and was created by the Sentinels that were ruling that timeline. He is more powerful than the generic mass-produced Sentinels, given that he is the most advanced known form of the machines, and has a number of superpowers as standard, including superhuman physical strength, formidable durability, regenerative healing powers, energy projection abilities, forcefield erection, magnetic control, the ability to interface with computers, shapeshifting (to the extent that he can appear completely human) and teleportation. In the comics, he followed Rachel Summers back from his own timeline to the main Marvel timeline (a backstory that could be altered, if necessary, in much the same way that Wolverine replaced Kitty Pryde as the time-traveller in the Days of Future Past movie) and faced off against the X-Men characters of that era/universe. In a movie universe where the X-Men think Sentinels are no more, the emergence of one more powerful than any they have faced before would be a terrifying and intriguing prospect.