Maybe Articuno could belong to a league opponent. That way Ash could use Hawlucha to defeat it.
While that would be interesting, considering the fact that Moltres and Zapdos were used to developed 2 of Ash's pokemon and evolve them, I wouldn't be that surprised if Articuno is used to developed one of Ash's pokemon which there is 3 possibilities for that and here they are:
1. We get an episode where Hawlucha battles Articuno and winds up learning a new move.
2. A 6th capture winds up getting caught and then we wind up getting an episode where the 6th capture winds up evolving in a battle against Articuno.
3. A 6th capture winds up getting caught and then we wind up getting an episode where the 6th capture winds up learning a new move in a battle against Articuno.
Now the thing is in order for Hawlucha to show off a new move that it learned in a battle against Articuno at the league, or for the 6th capture to learn a new move or evolve in a battle against Articuno and then use that in the league it would mean that Team Flare would have to interrupt the league and for the league to get postponed. I mean unless we get one of those 3 situations in a battle in the league however that would mean we would be having a situation where a pokemon winds up evolving or learning a new move during a league match.
The thing is I'm not sure if the writers would really want to have Articuno show up at the league, although at the same time I'm not sure why they would give a new move to Hawlucha, or a 6th capture or have the 6th capture evolve after the league is already over, sure it could happen but I'm not sure it will. Although whether Articuno is used to developed a 6th capture or Hawlucha, it seems like it would work out better if the league were to get interrupted by Team Flare and then postponed for 1-3 months thus allowing for a situation where Hawlucha or the 6th capture can use the new move they learned in their battle against Articuno at the league or for the 6th capture if it winds up evolving against Articuno to be able to be at it's peak when it battles in the league.
Obviously if by the time Articuno shows up if the only team change that happened was that Goodra return to the team then I would expect Hawlucha to be the one to battle Articuno and for the episode to be used to developed Hawlucha and for it to learn a new move, now if however Ash winds up getting a new capture then Articuno will probably be used to developed that pokemon.
Oh and If Team Flare does wind up interrupting the league then I'm guess it will probably happen in late July or in August.