Lo-fi? I like Lo-fi.
Mostly what you need, graphically, it seems, is palette adjustments--example, the rock tiles look as if they need more contrast, not to mention, the grass. Also, on larger tiles such as ground tiles don't be afraid to start using clusters rather than individual pixels to define shapes. Right now everything seems like noise, like the grass and rock tiles. Big no-no.
Unless your primary goal is to make it look like it's being viewed from above. Still you might want to take this into consideration.