Really like this style, I've donated to your cause and hope to see more tiles.
A great deal of flexibility could be added to this pack just by taking the same tiles and adding 2-3 more hues. For example, different hues of grass, dirt and stone, or a Lava hue for the water.
As for making packs game ready, I find it helpful if a pack has a white/semi-transparent "Selection" tile which fits over a single square. Turning it different colors in the game it can be used for many things, like movement range or attack range, or target/destination.