If you want to get better in the most general sense, buy a simple camera that is not in an odd format and that represents a 'blank canvas.' IMHO any 35mm, 645, 6x7 camera with a couple of lenses will do that. Stick to regular films and developers and go shooting.

I am not into the very long 617 format, but understand that others are, but I would say that irrespective of your liking for the format, it is probably not the best way to go about working on being a better photographer. For that I would recommend something much more portable and for which cost per shot is lower so you can shoot more. If you enjoy larger negs, 645/6x7 makes sense. In that case I would recommend the Bronica RF645 with an 45 and 65 or Mamiya 7 with maybe 3 lenses. Both can be shot casually in the hand or on a tripod for detailed landscape work. Both are dead simple to use and neither will limit you or weigh you down.