To develop a tileset that allows a player to terraform the landscape requires some work. There are some objectives you normally want to achieve. You want corners to blend properly, not a straight boundary. You want layers and a certain order to the stacking layers (e.g. grass on top of sand on top water). You want to draw it once during the game, so that you minimize processing power and maximize FPS.
It gets exponentially more difficult as you include more tile sets. As an example for 10 tile sets, the combination for the four corners would be 10x10x10x10 = 10,000 different tiles. In RPG Action, we can do it for you. Our algorithms will layer them, and generate all the different combination of blends. For example, the screenshot and sample auto-generate tile sheet (one of many) below.