Instead of complete “next level” mutation to e.g. Prime form (more armor, hp, movement etc.) e.g.
If player uses much snipers, normal form, but with snipers and more armor on head and arms, arned with snipers before, not only having e.g. just rifle-launcher all the time or pistol-shotgun
If player uses much explosives more armor only all over form, heavy like forms
If player uses close combat, faster and more close combat oriented forms
if they win heaven battle (no player response) adopting one fraction weapon to alien use, like technology theft from user side
Much much later it can overall all classess mutate to stronger form, BUT shorter and smaller in strength adjustment mutations, to be so called then current “big jumps”
Hope something similar can be developed. It would be a great great and grand in form child of past Xcom alien response curve of XCom 1994