Good point, although wasn't Black Cat combat/action as well? So I guess the support characters are mainly from currently running/recent manga with no tributes to the older stuff D
Psyren and Black Cat can really fit in here. I can understand if Train if boiled down to some sort of support character, but Ageha kicks ass.
I don't remember any hand to hand combat in Black Cat, it was all about guns. None of the characters in the roster use firearms, so Train Heartnet wouldn't be a good fit. He was shooting instead of hitting in ultimate Stars.
Some of the Ultimate Stars manga that didn't carry over to this game are combat manga, or at least feature it as a strong point. Kinnikuman, Ninku, Rokudenashi Blues, Houshin Engi (albeit that's more about trickery than fighting). I think Jungle King Tar-chan as well. Rokudenashi Blues and Kinnikuman seem to be the big sellers among those.
Rokudenashi Blues, Tarchan were only support characters in JUS, plus them and Ninku, Cobra are pretty old mangas and not as popular and known as Fist of the North Star or Nube. Newer generations may not know what they are, I can understand their exclusions. I think they excluded Kinnikuman because it's more of a sports manga than combat manga but yes, he would be an awesome addition. I have played the crap out of his game on Gamecube. So fun.