Hi Jemma,
I think Julia's just about covered it...except for Alaska...or maybe you can persuade JY to put together a tour which includes Chobe National Park in Botswana! Having said that, I guess with all the wildlife opportunities in their programme at the moment, they may not want to add another one.
I did the Borneo tour last year and saw lots of wildlife during our stay in the jungle lodges - Orangutans of course, Pygmy Elephants, Probiscus Monkeys, Hornbills and Kingfishers so I can recommend that. You will also see Orangutans and Sun Bears at the rehabilitation centre at Sepilok.
I did the 'Inspirational China' tour in 2014, but the only opportunity to see animals on that tour is at the Panda breeding centre at Chengdu. They are indeed very cute but I'm not sure that you could class them as 'wild'.
South Africa of course is a great starting point for seeing wildlife. Having visited Kruger National Park as part of JY's South Africa tour in 2012, I can recommend that as well.
I'm sure that other forum users will give you their thoughts on Namibia, the Kenyan safari tour, and tigers in Ranthambore in India, so you'll have lots to choose from.
Hope you come to a decision.
Cheers
Mike