I was doing the writing sort of roleplay recently (as opposed to the tabletop sort) and when the gang got fired up about creating an AU - as one does, in the throes of good RP - I proposed that the concept become a framed narrative instead.
Any time the group gets a wild hair up their ass about what if these characters met as children or what if this was a Weird West setting instead or what if they were all mermaids... Have the characters tell that story to each other. Occasionally have the characters comment on the AU as if they were talking out of character.
This keeps AU tangents from getting stale but let's you explore all the juicy possibilities and chase that creative high. Play until the What If question has been answered then resume the previous plot.
Honestly this probably works for tabletop games too, but I haven't tried it.
