It's actually mentioned in the guide that just came out, therefore I think we can talk about it in the open now
I was reading through the guide and I looked up the Hall of Heroes. It's unlocked once you get all nine Disney Originals (All we're missing is Jasmine at the time of this post) It said that you need a character who can fly or is fast because the door stays open for only a few seconds after you hit the button. But the guide doesn't mention that there's anything to find in there except that it's a hidden room.
Then again...the guide also mentions that if you get all 30 2.0 figs there's a secret toy (It doesn't say what it is) and it's not Luke's Landspeeder since that's for getting all the Marvel figs. But it seems like that special toy is going to appear in that empty space in the back.
So either the room holds nothing, or the super secret toy for getting everything.