With currently only one Bison that could be in danger of happening again.
I don't think Orangs would suit Whipsnade at all either, neither its location nor the enclosure. London got rid of them for several reasons that keeping them again at Whipsnade wouldn't address. IMO they should go down the amalgamation/pure Chimp route.
Would it be possible to move the European bison over and use their paddock for something else? Maybe they wouldn't like the open hill?