Based on the interview with Swen in PC Gamer, it looks to me like GM mode is one of the major reasons they are doing the Early Access (I guess they call it Early Access rather than Open Beta because you have to pay for the game to get Access?).
At any rate, here's the quote from the article:
"The super-secret Game Master mode is going to hit in Early Access also. That’s a very new thing, so we need to do a lot of testing with it. We’re really trying things there."
Most of the other features he describes as "tweaks" or "changes" -- game balance appears to be the other major item they're looking at, based on the article -- but this he outright says needs a lot of testing.
There are some really fantastic ideas in the game already, and given how good the Early Access looks, I think there's a lot of reason to be hopeful that a GM mode will be equally awesomesauce. It will doubtless require a lot of iteration to hit the sweet spot, but Larian has done a pretty brilliant job finding innovative and interesting ways to improve their games with every version, so it's something they're good at.
He specifically mentions forums as a source for feedback, so...
