I've always been steadfast against bringing back any characters. But just this once I'll play devils advocate. I could almost (and I emphasize almost) tolerate seeing the characters come back for one film, just for the fun of it. But only on two conditions.
First, the film would have to follow Gibson's script, and be given a hard "R" rating.
Second, (and most importantly) it would need to be cast with younger actors, like hellboy. I could tolerate a new film sliding in there to go off on a different path, but not with weaver and company. Bring in Hardy or some other younger but capable actor, and the same for Ripley and Newt's characters. That is something I could almost stand. But bringing back aging actors for the sake of fan service and a quick buck is something I'd rather not see happen.
Now, animated versions of Gibson and Ward's scripts with Weaver and others providing voiceovers would be something I could dig.