Didn't Leman Russ take his best men and run into the webway? They could easily have been stranded somewhere and unable to get back started rebuilding their chapter wherever they ended up.
They could be all dark and moody like the Dark Angels because they know the truth about what happened with Russ.
Or only the chapter master knows the dark secret and the rest is fighting to guard some "invaluable" relic not knowing what it is. Maybe they forsake the name of Space Wolves because something happened?
Maybe they are the first ever successor chapter to the Space wolves due to some internal conflict, maybe they can't take it anymore and NEED to hunt down Russ or at least find his corpse, the Chapter doesn't want to acknowledge this actions so X amount of marines are thrown out and form their own chapter to search for Russ.
The story oppertunities are endless