To fulfill the requirements of the EULA, cosmetics are one of the best donation benefits. I am not sure if particle effects and other cosmetics are just not supported yet with the SpongeAPI, but if they are, is anyone willing to create a cosmetic plugins with support for particle effects? The ability to have them surround the player as well as the ability to make trails out of them would be amazing.
In 1.9 support for the glowing effect and various colors of it, on both players and other entities, would be an amazing addition as well.
Edit: A command to set the color of leather armor in your hand would be great as well, possibly with pre-determined color names and/or RGB values(if possible).