Figured I'd try to work some of the clues/info for funsies, why not. Scattered thoughts:
Regarding Silence of the Lambs, the Clarice/Sheep clue is obviously very on the nose, but its also not the first time Legacy may be hinting at SotL in Shrek. In fact, he pretty explicitly references SotL going into Shrek as a possibility in a post way back in January around the time we heard Eilish house was canned. From what I've seen he rarely mentions explicit properties the way he did SotL unless its all but confirmed, and I think its a bit of an open secret the movie was planned for 29 but fell through due to rights issues. Either SotL going into Shrek was a called shot hilariously early, or he could have teased it this entire time without anyone noticing.
That being said, rights issues aren't insignificant, and SotL's does have them. As outlined by Neo below, SotL rights are split between two different companies who don't seem keen on working together in the movie/tv space. And as we've seen with Halloween, it only takes one party to refuse to play ball for a house to be scrapped. Then again, Universal's had the better part of the year to work through the rights issues, in addition to whatever headway they made in developing relationships with IP stakeholders during the HHN29 negotiation cycle. It's totally within the realm of possibility they were secured.
Speaking of Halloween though, there's been enough smoke (including the HHN2021 speculation map for Hollywood, hints around the HIII mask orders, etc.) that Universal may have popped the Mirimax bubble and managed to secure the HHN rights to Halloween Kills. Whether its due to attrition or Covid-19 desperation I don't know, but would love to hear about it if true. Additionally, while the first link on Legacy's haunt design page may point towards SotL, the final link to
Calendar Fear includes a paragraph on a potential Halloween room. Three trick-or-treaters, three HIII masks, it could be a potential hint as well:
Honestly, its hedging, but I can see either Silence of the Lambs or Halloween Kills sliding into Shrek, with Halloween III as runner-up. Both are properties with significant recent smoke, but with complex rights issues which could scuttle either or. Personally, I lean towards Halloween Kills. It's rumored for Hollywood already, and I can't imagine that 1. We'll see two significantly different headliner IPs on each coast (Beetlejuice and Silence of the Lambs for Orlando, and Exorcist and Halloween Kills/III for Hollywood), considering that 2. Universal Parks marketing and merchandising have made an effort to standardize advertising and merchandising for both coasts, even including taglines. Beetlejuice/Exorcist can't be helped, but I think that just means they double down on standardizing the rest of potentially sharable IPs.
If HK rights are still a problem, I think the fallback is the HIII with Michael to keep him in advertising. Sure there's potential rights issues there as well, but it seemed like Hollywood already navigated those and was moving full steam ahead until HK potentially became available, shifting spec and construction. Then again, I also wouldn't be shocked if its SotL does pop up (maybe Hollywood becomes HIII without Michael, which isn't marketable as a headliner without creating unfair expectations/brand confusion, freeing up Orlando to their own thing with SotL), but that's overall my thinking for now.
TL;DR: Silence of the Lambs and Halloween Kills are my biggest guesses, with HIII as the runner-up. Both are properties with significant hints coupled with significant rights issues, so I struggle to lean one way or the other. Personally, I'd lean Halloween Kills/Halloween III featuring Michael as I imagine Marketing and Merchandising want to standardize IPs as much as possible, and Halloween in some form is the way the wind is blowing in California.