Ok, so I've been playing a bit of Yuumi and although I don't feel as if she's weak (even though her win rate is abysmal) I still think this would be a good change so she isn't as dependent as she is now.
(Because seriously, she can't do shit on her own right now, she's almost easier to kill than a cannon minion)
So; make her Q range extend when she walks with it while not attached to a friend. Just like how Zoe Q or Aurelion Q fly longer when you walk with em Yuumi should have the same. It would give her just a little extra stuff that honestly won't really have an impact but just might help in one or two cases.