I believe that Jorb and Loftar have said that they do have plans for regrowing forest, and I understand that this involves minimum distances, so that it will remain possible to walk through even the old forests.
I'd like the new world to be more extensible than the old one. What I mean by this is not that the map should grow (Though that's awesome), but rather that new features could be added to it more easily.
For instance, if a new plant is added to the game, it would be nice if it could be spawned as if it had always been in the game, appearing in patches or however else they form in the new world.