ok, first of all it looks very neat so far, I would stay away from any more HD or high poly looking trees. IMO it has a nice WC3 feel at the moment, and the problem with replacing single things (e.g. trees or unit models) just make it feel off in my opinion, as you either stay with the WC3 feel (and if I remember right the terrains you posted for Land of Fables some time ago also look very Warcraft-ish and I guess you do not have many imports used there) or make it look 'better' with high poly models, but then you have to 'raise the bar' for everything and not only the trees.
As for the ground tiles, I would suggest you try the green/brown brick tiles from the ruins tileset, unless you do not have and space for more tiles.
EDIT: possible replacements for the current trees, would be the trees HerrDave posted to the model section lately