I think the other factions don't really exist in-game in a meaningful way so they probably can't have their standing raised/lowered. When I say "in a meaningful way" what I mean is like... the are Anam Sith grocers but not guards. So your standing with them really means nothing currently.
I'm fairly sure players are meant to drop heads and meat but currently don't. Based on that I'm guessing bandit heads and other heads such as walker heads may make it back in. I imagine there might be ways to raise reps based on turning in heads.