Well, they'd evolve to counter the advantages of the Orks, so for a first stage, maybe a good route would be stronger and more pointed outer pincers so they could better crack armor, and possibly a mild psychic field that could partially disrupt their connection to the WAAAGH!, which can be increased as it further evolves. The thing about metroids is evolution doesn't make them take on aspects of what they absorb the life of, but makes them a more deadly predator for the environment they find themselves in. So anything that could possibly make them more effective against Orks is fair game, so long as it's achievable through biological means. Metroids cannot evolve technological aspects (Metroid Prime/Dark Samus excepted, of course).