About the postgame Oak battle, that's actually a great idea. I'm quite fond of B/W and B2/W2 having super high-leveled postgame Trainer battles (I mean specifically, Cynthia, Morimoto, and N), and I don't really have anything planned like that for DarkViolet, but I may end up including it just because of your post.
Even though he gave all his starters away, I could just have his team without starters. Like, just having Tauros, Exeggutor, Arcanine, and Gyarados.