I wrote the Sponge port of BuycraftX, which centers around a GUI functionality. Little did I know, the Inventory API has not yet been implemented. Thus, I offer the following challenge:
Whoever contributes the most to the implementation of inventories in SpongeVanilla and/or SpongeForge (likely both through SpongeCommon) will be entitled to a $10 bounty.
The bounty can be claimed here (Bountysource).
This is purely an added bonus, and because I think Sponge developers are so amazing I might up that in the future. I have already become a $30 Patreon donor (funded purely with money I make off of my plugin(s)) and this is an added incentive for something that will be implemented no matter what.