The Scirra Store is a great opportunity to sell your assets and definitely pixel art is popular, we have some great tiles that are Zelda-esque in style as well as other lighter adventure games.
But there's definitely a few niche that remains unfilled.
1. Sci-Fi Post-Apocalyptic style, darker, faded, ala Fallout 1/2.
2. Darker RPG, Survival Horror.
Personally, I am looking for #1 or #2 tilesets, an example is from the game Mana World that uses modified RPG Maker tiles:
Notice the atmosphere of these tiles fit an eerie theme, or a darker RPG game.
Would be great if some of you skilled artists fill this niche, I'll definitely buy it on the Store here to support your creation.