I don't even care about Havok. Just put Cyclops and all his children from all the alternate realities together, and have them go on a road trip or something. Don't tell me that doesn't sound fun.
If the characters are overpowered, then invent an overpowered villain. The hypothetical Summers book should be about the relationship between the characters that make up the Summers clan. That's where the potential is. Fighting the villain of the day is of secondary concerns.