When you run out of tiles, press R to restart. Also, for the 2 groups of trees, it means 2 separate groups of trees, meaning it is divided by atleast one tile that does not have a tree on it. Although treeless tiles don't appear often, they do happen every once and awhile.
The best way to keep from accidentally combining two groups of trees is to refer to your tree group highlighter in the top left every now and then.
Also, I agree. Creeks or ponds would really add to the atmosphere of the game in my opinion. Maybe a lake tile you could keep expanding on until you surround it with ground pieces?
And with that, I think it'd be nice to have an optional feature of being able to name things, whether it's the villages, big forests, stretches of field, or the rail roads. However I think with that there'd have to be a set of requirements to be met before being able to name something, for example a village can only be nameable after consisting of 5 houses.