I like your mockup. First thing I'd change would be the sprites. I really like the concept, but your shading is too flat, not enough contrasty, too much colors and not good lightsource (and outline-handling). I tried a edit.

I used the same colors as in your sprite, so there is a lot of optimization (tinting wise), but I hope it's enough for a quick edit. I lowered the colorcount (no idea high the original was ->photoshop) by just using more contrasted colors.
You used a lightsource from the left, which is not a good decision for gamesprites. Mostly sprites get flipped ingame (timesaver) so when you have a left/right lightsource, the lightsource flips with the sprite. You can prevent this effect by using a lightsource from the top. In my edit I tried to shade it this way and treated the outline the same way. Your outline is getting much better compared with the old broken black selout, but it's not enough consistent. On the head you have the a darker outline on the left side and a lighter on the right, but a lightsource from the left, same thing on the stick. Just outline according to the lightsource.
I think the background works this way, but it needs antialiasing. Made a example:

I don't have much experience in AA and I think I over did it.. but maybe you can give the illusion of depth when you make the background slightly blured. And I suggest to use round shapes instead of geometrical (or you have the same style on the foreground too).
All in all a great a great start. Keep pushing!