Decimation was an interesting idea, but Marvel really failed at actually executing it. Millions of faceless mutants lost their gifts along with a handful of minor characters, and it really didn't matter. Polaris was decimated and promptly got her powers back. Marvel faked us out with the idea Iceman had been decimated. Magneto was actually decimated, but he too eventually got his powers back without any interesting stories of a decimated Magneto being told. If Decimation had any breakout stars, they were Rictor and Danielle Moonstar. No offense to those characters, but that's pretty sad.
What is the point of having a bold idea like Decimation but not having the balls to do anything with it that matters? Here are some mutants I think Marvel should have decimated.
If something like Decimation is going to matter, you have to hit one of the original X-Men. There's no better way to show you're serious about the story. Decimating either Cyclops and Beast would be stupid. It would do more harm than good to the characters. But it could have been good for Warren Worthington. The character had been treading water since Marvel completed its reversal of all the Apocalypse did to him. Angel was the most boring of the original X-Men. Forcing him to lose his wings again would have done great things for the character as it easily could have led to him becoming Archangel again. Apocalypse reclaiming Warren as his horseman of death would have been far more interesting than the pathetic gathering of horsemen he later showed up with. Warren has since become Archangel again in X-Force, but doing this earlier would have given some much needed weight to Decimation.
Exodus isn't a bad character, but he has one serious problem. He's Magneto-lite. He's always going to be Magneto-lite, and there's really no room in the X-Universe for a Magneto-lite. His only role in X-villainy is to follow or fill-in for Magneto. It's a role that has been thoroughly played out. Exodus really has no room to grow as a character the way he is. His most likely future is to be killed off someday to show the toughness of some new threat. There could have been a lot of potential to a decimated Exodus. How would someone as unstable and in need of a cause as him react to no longer being a mutant? He and his new Acolytes could have been remade as a new threat to mutantkind, wanting to wipe out those who unfairly kept their gifts. Exodus, who was actually a holy crusader, would make a much more interesting leader of a cult of religious fanatics seeking to wipe out mutants than what we get from the Purifiers.
Cyclops' little brother has never had much of his own going on. Havok is defined by his brother, and most of the things done with the character make him seem like Cyclops-lite or the lesser together of the Summers brothers. That's not much of a character to have. I know I'm being hard on the guy, but just think about what two of the main highlights of his history have been. He led a team he inherited from his brother, and he lived in an alternate universe where he basically was his brother. Currently, he's stolen his dad's role as space pirate, which doesn't particularly fit the character. Decimating Havok would have actually forced him to go become his own character, and would the X-universe honestly have lost anything? It has been a long time since Havok has really contributed anything to the X-books. With his government connections from X-Factor, the character could have had life outside of being a mutant. He could have become an agent of S.H.I.E.L.D. or even an agent of S.W.O.R.D. with his connection to the Starjammers. Controversially, he could have been recruited to lead the Sentinel O*N*E unit meant to protect what was left of mutantkind. For once, Havok could have had his own unique thing going on.