Considering how last years theme reveal was a dissapointment, it wouldn't surprise me that the whole Evil Encore is just a fancy way of showing of repeat houses/icons/scare zones etc.
As for CoS returning for a 5th year (It debuted in 2012), well it's "better" they announced it right off the bat. If they waited till later eyes would've probably rolled out of sockets.
It's totally understandable Unearthed is coming back since it's only two years old, though to also no surprise it's just "Unearthed" and not "Unearthed: Scarlett's Revenge". Take it she didn't leave much of a impact to justify keeping the name on the house.
It'll be interesting to see what houses return and which ones won't, since if we're going by the oldest ones out, Zombie Mortuary is easily the oldest house at the event since it's been there since 2011. However there's a strange catch-22 since I've seen in a lot of reports last year that people loved that house, even more then the newer ones! So it could end up staying due to the popularity.
The next oldest houses are The Basement and Death Water Bayou (Both debuted in 2013, making them one year younger then CoS.), followed by Dead Fall and ZCU that came out two years ago.