I'm not sure if we had this already, who are the mutants who had their 2nd mutations already? (Not sure if my list is right too.)

White Queen - Diamond Form

Jean-stronger tk and tp, being able to tap on the Phoenix Force

Beast- Physical mutation to a more beastial/catlike form

Dazzler-immortality?

Why aren't the other old mutants having their 2nd mutations yet?

what 2nd mutation would you like ton give

Storm

Wolverine

Magneto

Cyclops

i think mystique had her powers mutate to a much stronger degree.

i dont think EVERY mutant gets a second mutation just a few, and i think Mystique and Kid Omega both have had one

I think you're right there. Not EVERY mutant gets a secondary mutation. Thats more up to the writers if they wanna spice up the character/s.

The Phoenix Five and Magneto have gone through some changes lately. While they seem to have lost some control over their powers, both Magick and Cyclops seem to have alterations and possibly even advancements on their powers. Whether Cyclops' increasing power levels are part of a secondary mutation or not remains to be seen though. Of the classic X-Men, Cyclops has been unchanged for decades. Jean and Beast as mentioned. Angel becoming Archangel. Iceman exploring his full potential. So i do hope that the changes being made to Cyclops power is some form of secondary mutation, or at least something similar.

Another character who i think could do with a change is Polaris. While Magnetism is cool, Magneto already owns it. so seeing a variation of the power could be interesting.