Basically, a room in the ATH with stuffed, scarecrow-like reproductions of monsters from the clockworks. Just need one sample from each family, preferably in 3 different versions such that the damage inflicted to them by attacks is roughly in line with what you'd see in average T1, T2, and T3 depths. It'd also be nice to have a scarecrow version of vana, seeing as he's not a part of any monster class.
It's been suggested before in various forms, but I figured I'd put my version here.
I like the idea of there being a scarecrow/puppet of the different monsters (except for the rare ones like impostocube, sloom, etc). However you must remember that in the ATH you have a very limited amount of HP. A hit from a T3 scaled monster would most likely kill you.
Also, bosses like Vana should not be included as they are bosses and should only be seen, fought, and experienced by players who can make it to their respective lairs. Or else you'll lose a lot of the fun. I mean I would not have enjoyed my first vana run if I had already seen or "dealt" with him already in the ATH. I would not have had the rush of fighting this massive beast for the first time.