Going with the idea of a dynamic heightmap it would be cool if the texture changed if you walked on it too much, not enough etc.

for example:
You have a city but the texture is grass. What do you do? Well, every time someone runs across a certain area it adds a slight shade to the dirt texture. This eventually will overpass the grass and make a dirt path. Over time, if the path is not used it will slowly fade back to grass. Should grass not be stepped on for a long enough period it will become overgrown and weeds will pop up (killing crops and increasing the chance of a monster spawn). Should a dirt path be trod on to long it will eventually become gravel which will eventually become paving stones etc. This way, the terrain would change dynamically without the players having to do anything! This should apply to textures placed by the bucket as well so you don't run across a popular city with overgrown grass streets or something like that.

Not sure how easy this would be to implement or the load it would place on the server but it would be really cool!

It would be cool indeed (The Settlers had (still has?) such a system, and how old is that game? O.o), but it would make the current texture placement less useful.

The game as it is now is perfect for creating, but I don't know how the gameplay could go.
I think the game would depend too much on roleplay, since players basically make the world. Are there any plans for this?