I'll try my best. As I said, this is based on information given in zoo news, information from Bill Hall and a shaded map showing the biome areas I once saw.
BRITISH WOODLAND:
Will be based in what used to the amusement area along with extra surrounding woodland. Will comprise of walks consisting of native plants and information on Marwell's in-situ conservation, how visitors can discover and help local widlife, and will exhibit some appropiate animals like Dormice.
SOUTH AMERICAN COAST:
Centered around the current penguin enclosure.
EAST AFRICAN SAVANNAH:
Main highlight will be the African Valley, housing rhino, giraffe, ostrich, zebra, etc. with a safari ride of some kind being built through it. Hunting Dogs likely to return, as well as the possibility of lions.
AFRICAN SEMI-DESERT:
Centered around the current grevy's zebra/oryx paddock.
AFRICAN ARIDLANDS:
Centered around, well, Aridlands!
EURASIAN STEPPE:
Prezwalski's Horse, Bactrian Camel, etc. Possible locations include their current paddock or the one currently holding Ankole.
CONGO RAINFOREST:
Includes the current Okapi complex. Will take up the whole area between Aridlands and the Hall Gardens, and will feature a gorilla exhibit among other things.
AUSTRALIAN BUSH:
Includes the current Bush Walk & Encounter Village. Nearly complete, with phase two, a paddock containing the emus and kangaroos being finished this year.
EURASIAN BOREAL FOREST:
Centred around the current sitatunga paddock. The Amur Tigers & Leopards will likely be moved here.
INDONESIAN FOREST:
Centred around Life in the Trees. Possibility of Komodo Dragons.
MADAGASCARN FOREST:
Centred arond World of Lemurs. Pretty much there already.
HIMALAYAN CLOUD FOREST:
Centered around Roof of the World. Again, pretty much done with the possibility of only one or two additions.
WEST AFRICAN WETLANDS:
Centered around the Semi-Aquatic Mammal House and Flamingo Enclosures.
SOUTH AMERICAN GRASSLAND:
Centered around the current rhea/capybara paddock. Possibility of Maned Wolves returning.
SOUTH AMERICAN RAINFOREST:
Centered around the current Entrance & Gift Shop area. These will be relocated to the top of Treetops hill, and the Ark Gift shop would likely become the new tropical house.
This is what I know so far. Of course, changes could be made at any time.
