Please use caution when talking about map "Tiles". It's confusing even in the Terrain Editor Forum. The map "Tiles" you speak of are terrain "Textures".
And, yes as mentioned above if you are up to the task I do believe it has been mentioned that HTC is looking for new terrain textures. But, I have avoided that because you or HTC would have to come up with matching objects (trees, buildings, etc.).
Keep in mind that there are currently 11 main textures (4 of which have not appeared on any MA map that I know of yet) with a total of 4 per category. That's a total of 44 textures in a complete set.