pixelmon:tree - A player could interact with the block to "cut" it, making it invisible and allowing a player to pass though it temporarily. This would be very cool to recreate the cut bushes from the original games for quests. I've been getting around the lack of this feature by using tags and command blocks to set the block to air temporarily when a tagged player is near. If this were implemented, it would be nice to be able to restrict the "cut" mechanic on trees until a particular quest is completed (maybe just a permission node?)
pixelmon:boulder - A player could interact with the block to "break it", making it invisible and giving them a random item or an encounter with a random ground or rock Pokemon. This would recreate the Rock Smash mechanics from the original games.