Few suggestions on the Better Grass Spawner behavoir:
Add other Pixelmon Grass textures to the Better Grass Spawner
Add an option to make it invisible (in which case it would behave like a Barrier, becoming visible only when the player is holding a Better Grass Spawner item)
Make it possible to place it on water blocks (like lilypads), allowing map makers to create random encounters on water surface
Add a Zone Wand compatibility, allowing a player to copy a setted Better Grass Spawner, select an area with the Zone Wand, and fill the whole area with that pre-setted Grass.
I would like to second what Cavans99 suggests above. I was about to make my own similar suggestion issue but I see he already made one. I would like to add 1.5 extra bullet points.
Allow Better Grass Spawner to specify weather condition for spawn and Day/Night etc.
In addition to the lilypads suggested, also just allow an invisible/passable spawner to be placed on water surface.
Few suggestions on the Better Grass Spawner behavoir: