I actually expect the Ash vs Serena battle to be interrupted somehow, although not necessarily by Serena herself. Team Rocket could appear during the showcase performance thing and cause the battle to have no conclusion and instead Ash and Serena might just team up to defeat them. ^^
Now we know Serena isn't actually leaving in this episode, i don't think Serena will quit the battle and leave now, i expect it may be interrupted unfortunately too
I'd prefer if the battle wasn't called off/interrupted since i want the focus on Pikachu and Braixen battling here, it's awesome to see my two favorite middle stage starter Pokemon battling it out (i know Pikachu isn't a 'starter' technically, but...he's Ash's starter haha xD). PLEASE let there be a conclusion-even if Serena loses it's fine by me. In her last battle it wasn't called off in the tag battle so it gives me some hope this'll be concluded...
Maybe the unexpected event leads to Serena doubting her goal or such because of the title saying 'Serena's Choice' like she wasn't sure what she wanted to do again, so Ash offers to cheer her up with a battle. Pikachu seems like he has the upperhand (which is very likely got to admit xD), and Serena is on the verge of giving up-BUT when Braixen senses her trainer giving up, she evolves or learns new move (Psychic please!!) and reminds Serena 'not to give up', like what Ash said to her and then she decides to not give up on her dream and keep working at it, battle concludes with being called off somehow (though i'd rather it wasn't...) or Serena losing, but only remotivates her not to give up on her goal. If Braixen learned a new move or evolved for that reason or similar i'd be happy with that (the only way i'd be happy for Braixen to evolve tbh...)
I say this because imo i see Braixen as a reflection of Serena herself in Pokemon form (since they're so alike):
Fennekin: Serena didn't have any goal in sight/no idea what to do/trying to find her interest
Braixen: Fennekin evolved after Serena found/started her goal (Kalos Queen) and is actively working at trying to achieve it
Probably won't happen but i'm only speculating haha, even if above doesn't happen, seeing Serena battle with her starter is always fun to see ^^