No idea, although some of the subspecies are now considered endangered.
I think there are grounds to bring in quite a few North American species not commonly found in the UK.
I would like to see more Puma, Bison, Bobcat, Lynx, Grizzly (Kodiak especially), Black Bear, Wolverine, Moose/Elk, Wapiti, Bighorn, Rocky Mountain Goat, even down to Groundhogs, Prairie Dogs and Marmots.
They would be the basis of a fairly good collection on their own!
However, specific to Marwell, since Meerkat seemed to supplant the majority of small carnivore spaces (a sadly common issue for a long time now), it would be a great move to have some active species that aren't too shy. The Coati family "pit" always had a crowd around it watching their antics.
Even the Red Panda always had a crowd simply trying to find them in the trees. The ones at Longleat seem a lot more showy and confident.
Doesn't need to be something big or an expensive exhibit, just something interesting.
I'm sure there must be surplus animals that could fill the empty spaces up for next to nothing, cost wise.