I most want to see either a full relaunch of CCL or an Algernon themed remake of cred street (having a neutral theme will also give them a lot of room to try some cool themes without having to worry about the rest of the area)
It would also be a good excuse to open House of Monsters as a permanent pay-per-ride attraction!
Some cool ideas! I would personally love to see a relaunch of Cloud Cuckoo Land as I think the theme has potential but will only do well with the right execution!
My only concern with a "neutral" theme as you called it would be the fact that having lots of different themes under that may seem a bit random to the every day park guest.
AT would not want to open House of Monsters all year as it would cost themselves money as it wouldn't be as exclusive to Scarefest and AT would need to hire actors all year round. I have heard that it was a very well thought of attraction though!
As I said though, a well thought out relaunch of Cloud Cuckoo Land would be fantastic idea!