Elephant seals for me...
Forgot to mention 2 species which have been kept with some succes but are now kept no-where anymore, not even in their native West Africa : both species of Rock fowl !
Indri, pygmy hog and stary nosed mole
Siberian Snow Sheep? I imagine Russia must have a few in Zoos.
Dorado is kept at some Aquariums all over the World I belive, same with Mola Mola. Marlin/Sailfish was also kept in Japanese Aquariums, don´t know if still there too, and the new Singapore Marine Life Park might also had sailfish/marlin to its collection.maybe species like dorado, flyingfish, mola, and maybe even certain marlin, might be able to be kept in captivity.
And while I'm at it, don't some people want to see deep-water species like anglerfish in a high-tech pressured tank sometime? I certainly would love to see something like that.
I came here to say this, only I set my sights a little higher: if this becomes a reality in my lifetime, it will excite me more than any other animal ever could.