I'd like to see more Apricorn Balls made from colors not native to Johto (like purple and orange). Orange could be used for a Sun Ball (a Sun Stone version of the Moon Ball), but I've not got any ideas for the Purple Apricorn. Maybe even rare-colored Apricorns like Gold and Rainbow could exist and be turned into Poké Balls with really cool effects (like maxing-out a Pokémon's IVs or changing its color), but these would be incredibly hard to find (maybe a 1/4000 chance from any Apricorn Tree).
Here's an idea I had from the distant past:
Name: Egg Ball / Cradle Ball
Appearance: A ball made from the discarded shell of a hatched Pokémon. The opening is zigzagged.
Effect: The original idea was to explain hatched Pokémon magically getting their own Poké Balls (all hatched Pokémon would be in this Ball), but since many players enjoy the new-ish ability to pass-down Poké Balls via breeding, I figure it would be better to make it increase the catch rate of Baby Pokémon found in the wild instead.