Lets say she retains her usual appearances in the plot, the only difference being that instead of her being the Shalour Gym Leader, its old man Gurrkin. She still meets you a few times to battle prior, still fights your first mega battle, still foists a Lucario on you etc etc.

Then when you're past the E4 she reveals herself again at full strength, saying she saw potential in you and wanted another strong Mega-trainer to fight and now its time to see how far you've come, etc etc.

She could run a team something like:

Mienshao
Pangoro (or Scrafty, but a Kalos mon seems more likely)
Toxicroak
Hawlucha
Machamp
Mega Lucario

Feels like it'd be at least a little tougher than Diantha was, especially if she had reasonable movesets, along with being better tied into the story.