2. Apocalypse
Though I suspect that Apocalypse will be saved for the third X-Men prequel, and instead we'll probably get saddled with a precursor villain, how awesome would it be to finally see Apocalypse on the big screen? Apocalypse is one of the very first mutants to have ever resided on Earth, starting out in Ancient Egypt, and through the centuries, amassing a wealth of power and a legion of fans. A Darwinist, he believes in selecting the strongest of the strong to help him rule over everybody else. The technology he has accrued over the centuries makes him practically impossible to kill outright - hence he could return for a third film if briefly bested at the end of this one - and it also means that he can jump in and out of various pockets of time completely at will. He would tie effortlessly into the Sentinel storyline, wanting to apply his Darwinist values to the mutants and ensure that only the cream of the crop make it through the onslaught. Given that Apocalypse is able to change his form, this again would prove no problem for Dinklage.