I doubt Lorde would surprise release anything. She's not the artist who's able to pull that off, especially after being out of the limelight for so long.
"The return of a star" would lead me to believe that it's probably Katy, because even though Ed is a star, his release wouldn't really be a "surprise." The only thing is that people don't know if he's dropping his entire album on Friday or if he's just dropping the lead single.
Honestly, though, the only other two options I could see would be Taylor Swift or P!nk. To be honest, I don't really see Taylor straying from her usual release schedule too much (at least in terms of her October releases—I know she typically releases her albums two years apart, except for this next one). As for P!nk, it would be plausible, but I can't see her attempting a surprise release, and having just had a kid, I can't imagine she'd want to release an album in any capacity immediately after. She's inevitably going to have to promote her stuff somehow and she wouldn't just stop being a mom to do that.
Therefore, Katy seems like the most plausible answer for right now. Although I feel like HDD has teased us like this before and then it ends up being someone nobody cares about. I'm not getting my hopes up yet.