A couple of thoughts on Wally:
Let his Roselia and Magneton evolve. This is fairly self-explanatory.
As for Gallade / Gardevoir, I think it'd be interesting if it was random. Like, he encounters his Ralts and if its a boy, he ends up with a Gallade, and if its a girl, he ends up with a Gardevoir. I know male Gardevoir exist, but we'll just say he won't evolve one. The reason being allowing for that would tip the scales in Gardevoir's favor.
Get rid of his Delcatty. Delcatty sucks. There are two Pokemon that come to mind to replace it - Slaking or Linoone. Both for the same reason - it would be semi-subtle nod to Norman, whom he owes for being able to start his journey.
Don't add any mid-game fights with him. Honestly, I really like the way he comes out of nowhere at the end of the game - the first time you play, you've long since forgotten about him by then. And, even if you haven't, you completely counted him out - he's a sickly trainer with a weak Ralts, and that's all he'll ever be.
And then, he shows up with decently strong Pokemon and surprises you.
I think it'd be cool if other NPCs talked about him, not by name, or else it would completely spoil it, but I think it'd be cool if some trainers talked about how you beat them even easier than "that talented pale kid" or things to that affect.
Also, and this isn't exclusive to Wally, but I think major characters should have Secret Bases you can find. Maybe they could be tied into getting their Match Call / Trainer Eyes information? They don't necessarily have to be in them, but it should be obvious who it belongs to in some way.