Actually, one of them killing, say a Purifier, to save another mutant, or a teammate, would be interesting for the team dynamics. I remember wanting to see Marvel deal with the emotional, and social, stress Hellion's damn near killing of Omega Sentinel would caused him, and those close to him....but then we got Age of X. AoX was cool, but was still just a distraction from actually dealing with the issues these characters were actually going thru.
I didn't think of Hope Summers, but you bringing her up does make a lot of sense. Seeing Pixie able to switch between Nightmare for offense, and a more twisted personality, and her default form, similar to Illyana, would be pretty cool.
If Hellion joined/formed this group, I feel that Mercury and Rockslide would have to join him; they're like his sister and brother. It's a shame writers have forgotten about his bro-sis relationship with Cessily.