This is my first post here on these forums so please excuse my ignorance!
I have been developing Bukkit/Spigot plugins for more than two years now and I think I have an “average” amount of experience in developing small plugins with standard features. However, one thing that I’m missing when I look at Sponge is the “MapRenderer” functionality available in Bukkit/Spigot.
This MapRenderer allows you to draw shapes and text to a canvas which would be loaded to the players holding maps. You can register a MapRenderer to one or more maps. This allows plugins to create custom map based displays and signs.
If it’s not, it will be eventually. Keep in mind, Sponge is still in development! (I don’t personally know of any such functionality, but it’s possible that it does indeed exist).