While I don't have an exactly spelled out vision (ie: I didn't sit down and make an actual design document), I would like it to eventually become the GIMP of digital terrain. Here are some things that I eventually want terraform to have (if you're not sure what I mean take a look at the GIMP (http://www.gimp.org), where many of these ideas come from):
move to a multi-window GUI to allow editing of multiple height fields simultaneously
implement selection so that you can select a subset and work on that only
ability to 'draw' your height field with different 'brushes'
provide the ability to 'merge' different height fields