The GS Ball saga is the one thing that I think should be revisited briefly, given how long it's been since we've heard about it. I remember there being so much focus on that mysterious ball that nobody could open, but then all of a sudden the thing was handed over to Kurt in Azalea Town and we've never heard about it since (supposedly because of Pokémon 4Ever).
It's funny how certain things that were a big deal at one point, but eventually, the hype quiets down and we never hear about it again.