The only one I would be willing to migrate to is vBulletin 4.
Well, vBulletin is a paid platform, you can always get the 'free' version still...
If you prefer paid one, I hear a lot of good things from xenforo, less bloated, simpler interface, not to mention cheaper.
Comparing them before choosing one might be a good idea (IPB is still bad though)
Back on the free forum, SMF in my opinion is really great and really light weight, the biggest downside there are little support on mods and themes hence the current problem. People seems to prefer myBB but I don't know, phpBB is bloated don't use that.
But in the end a simple mobile interface is all we need, which is something that can be done with some tweaking in css. How about posting the theme to github or something so members can give you a pull request so the forum itself is made by community.
Or you can follow dfg and install vanilla.