Just a suggestion, I'm sure there are people with far greater experience on this forum who could provide better advice but here is my two cents: You could put it in a bigger pot, put it by a window or outside, and let it grow feral for a year or two. This would give the plant a chance to become healthy, thick, and strong enough to be trained into any shape you like.

If the roots on a tree quit growing then the tree will die. The tree is probably root bound. It would be good to pull it up and trim the roots. This should be done every 2 to 3 years to keep it healthy also I would suggest a good bonsai book. Unfortunately I don't know of any.