Wisteria, being vines, love to climb. If you'd like a thick and twisted trunk, there's no better way than to put it in the ground and give it something to climb on. It will thicken by an inch or two a year. I would plant the trunk at an angle and cut it back to the first leaf set and let it start over just twining around a post.
Cut it back again each year a little less, and in 5 years you would have something really respectable.
If you don't want to do that you can develop it on and make a nice bonsai, but even with a think trunk you will want movement to make the trunk more graceful, so I would again cut it back to that first leaf set and get some movement in the trunk.
Hope that helps.
We have a real lack of wisteria in the gallery, but here is one. IMHO, there is not enough movement in the trunk to make a nice tree without flowers. If you didn't cut yours back, but say grew it on, it might look something like this in time. In bloom, no one is looking at the flowers, anyway.
