If the British are concerned about manpower in OTL the Blackfoot straight up volunteered to go after the Cree. It wouldn't be a stretch to arm them and give them a free pass to go south.
The Metis I might do something, but their big gripes with the federal government hadn't really started yet so I'm not super sure what they'd even have to fight about. There's still a lot of buffalo so they're not even a settled people yet at this stage prior to the buffalo wars.
Now I never said who they might cause trouble for though rest assured, at least one of the above mentioned will be showing up at some point. Though to be fair I've already mentioned the Dakota being something of a problem.