Alright, here. we. go!
With HHN releasing the info of them doing reimagined haunts from years past, that'll make speculation better for me. When it comes to speculating new IP's, I'm not gonna' really delve into that since until we get any solid info on what IP's could be coming, it could be almost anything. (An that is what usually dives into throwing anything at the wall.)
Now with the reimagined haunts angle, that's more fun to speculate!
Scarecrow - Well this was going to be the obvious first choice due to the reception of S:TR in 27, though it looks like HHN is clearly getting the obvious out of the way. Considering I was just a year shy of experiencing this (An very foolishly said it looked repetitive based off a house POV, something which I'll never do again after experiencing 28 and 29 first hand.), I'm looking forward to it!
H.R. Bloodengutz/Slaughter Sinema - If there's any house other than Scarecrow which would have a huge clamoring to return, it's H.R. Bloodengutz. It's certainly a theme that can be redone again and again, case in point Slaughter Sinema is certainly in the same kind of vein with the schlocky B-Movies. Because of that, you could very well combine the two, with H.R. hosting a movie marathon of sorts.
The In Between - It's been a little while since HHN has done a 3D house (and the last one was quite possibly one of the worst), so I can see that not only returning, but from what's often considered the BEST 3D house they've ever done!
Another yearbook mash-up house? - Monsters & Mayhem certainly showed the formula on how to do this type of house right, I can see them doing a new version with it being the 30th.