There are a lot of ideas floating around about the use for the old reptile house. One is a ZSL museum/history of as you said. ZSL has so many really interesting items hidden away in storage which could make a great exhibit. Another idea is to take there Vets in Action half-term event they have run for a number of years now (kids dress up as vets and do pretend exams on soft toys), expand it and make it permanent. Similar to the whole Little City/KidZania role play events you see around the country now days just Zoo focused. Dress up as a vet, keeper, role play etc
But the Zoo will be very aware of the perception of entering and the first thing you see is two closed buildings. Not a good look when a average family may have just spent the best part of £100 on entering.
In other news, 3x warthogs have arrived, either 3 female or 2 female and 1 male. Should be on show this weekend.