I think a city alone can be enjoyable, but a city inside a nice environment is even better.
The trees idea is interesting, we should be carefull not to plagarize too much any already existing Ages though. They say imitation is the best form of flattery but I'm still sure we can come up with something original.
Also of concern is the performance aspect (ie: the number of polygons & textures our computers will have to display). The city itself will already be quite demanding (especially as new shells will be filled). Building lots of new stuff outside of it would be 'risky'.
That said here are a couple idea to expand on it: let's say the city is atop some huge choped tree. (those who've played World of Warcraft: think of the night elves land). The tree could be ever-growing to accomodate for the city ever-expanding nature. Since we can't go out of the city itself (we can't, right?) we would never see the tree itself and thus wouldn't need to create it. Outside, we could see other huge trees in the distance far in the fog..
Surrounding the city we could see the tree's bark, just a ring all around the city 10/20 feet high. (not sure if my description is clear though)