I know overworld sprites can be hacked. I wonder if it's possible to hack the battle sprites.
The battle sprites, especially the front sprites have animation.
Emerald, Diamond, Pearl, Platinum, HeartGold, and SoulSilver have 2 frames of animation and uses scaling and rotation.
Crystal has more than 2 frames of animation.
Lei Dian Huang: Bi Ka Qiu Chuan Shuo alternate between Gold sprites and Silver sprites, implying that it was released a month after Emerald.
If we can hack the sprites in Crystal, it opens up to having extensive hacks for that game.