Quote Originally Posted by Yora View Post
A proficiency bonus for only things that are not attack rolls should be very easy to do, though.
I mean, for noncombat stuff you can literally pull the 5e Proficiency rules out, drop them into anything B/X related, and they'll just work. You have caused massive issues for the Thief, but as long as nobody's playing one it'll work. I do recommend letting Thief abilities lean towards the improbable in that case.

Plus if you want high is good AC both Lamentations of the Flange Princess and Basic Fantasy use it, although on southerly different scales. Although LotFP is slowly moving away from B/X, it's main delay on a full new magic'system send to be getting that referee book out.