Is there anything being revealed on the day like a theme or something to make this year different to all the other years? 

From what I saw from the previous year post (https://itch.io/jam/7drl-challenge-2019), there is no theme.

They also encourage you to start from pre existing engine or game project. I started working on a turn based combat system, so I can spend the timeframe trying roguelike stuffs :)

Cheers!

There is no theme.