the notorious enemy could be just about anyone
heck given that LOS made cornell a villain it could even be a character who was previously a protagonist, but I doubt it's Satan because Cox has said that both DLC take place before the epilogue.
This notorious enemy could definitely be anyone. Seeing how there is some mention of a time machine, it could be Galamoth. After all, the Galamoth soul in AoS allows Soma to recognize areas where time has stopped. In other words, it makes him immune to time/space magic. Given that, it is fair to assume that Galamoth has some sort of time manipulation capability (though he didn't use it in SotN).
So, if it is Galamoth, the time machine could open up a some sort of time portal where you battle Galamoth. You may also have to use it to solve some puzzles.
If it isn't Galamoth, then who ever it is could be trapped in an inaccessible place and the only way to get there is to travel through time to open up the way.
This is all pure speculation. I guess we will see next month.