The only artist I know who has a fully done set of pixel art for rpgs is finalbossblues:

https://finalbossblues.itch.io/

He's got tons of characters, all animated (at least walking), and tons of tilesets. One could make an extensive rpg with all his assets. Does anyone know of other artists who similarly have tons of assets all done with the same theme, as if to be put into one game?

I can think of two free asset sets that also have quite a lot of stuff to them:

First would be the 2D stuff by kenney   : 
https://www.kenney.nl/assets?q=2d

https://opengameart.org/

and the second one is the liberated pixel cup, a large open group project where many artists try to create ressources following the same style guidelines, so they can all be used together.