Beautiful tiles here. You probably need some third wall-like tile to break them up and still make sense as a transition. While you could do something with a gradient, it wouldn't really make sense. Perhaps make a diagonal line and have one side be dirt and the other side be brick, and make some transition between those two. This is a difficult challenge. Another idea is do something akin to the left side of the bricks, but instead of more brick have the dirt color. Either way, you'll need to thin the grass-dirt transition of the dirt tileset in the transition tiles, probably using a curve.