If you are capable of making a PBR game in 7 days there is nothing stopping you, Epics tools are a great place to start, I certainly wouldnt tell people to dream small I believe scope is important though. Honestly though if youre going to go that way why not go all the way and find AAA backing rather than competing in a GameJam, Im sure there will be professionals about but I really dont think Blizzard are going to send a team over or anything, you know what I mean?
Its basically like when someone tells you they want to make the next GTA game, the first question that comes to mind is you and what army? 
It’s actually why I was originally against the idea of the Jam, I didn’t believe the limited scope could really let people flesh out a game design decently, that the games produced were more tokens, representations of games rather than a full complete product. I’m not so sure that’s a bad thing and I’m certainly more prepared for this Jam.
Goodluck 