When TW artists use the established cartoony, solid-colors style, it's because they're making a choice to go in the direction of internal consistency. It's not a statement that that's the only direction ever worth going in, or that nobody should experiment. Just that TW's art style is fundamentally solid and a very good match for the gameplay, and there's a lot to be said for building up what's already solid and well-thought-out instead of going in lots of different new directions. But, it's a free game and you've got great free graphics software to choose from; you couldn't be less restrained from making a cool photorealistic tileset, putting up a server and being perfectly successful if it works well.
I think you'll find one challenge is that having lots of more detailed tiles on the screen can create a blurry/unpleasant-to-look-at effect for a player moving at high speed, especially at high resolution. It's a subtle thing that the stock tilesets avoid very well.