We haven't brought back the passenger pigeon or the Carolina Parakeet yet and both went extinct at the Cincinnati Zoo in the early 20th Century.
>The Great Passenger Pigeon Comeback has set ambitious goals to hatch the first generation of new Passenger Pigeons before 2025 and begin trial wild releases in the following 15 years. http://reviverestore.org/projects/the-great-passenger-pigeon...
The zoo at this point were just recording the last survivors for posterity's sake.
So do penguins, gulls and terns, yet they can be bred in zoos. Also, the big colonies of the passenger pigeon mostly like where a phenomenon of the 19th century (possibly human induced), the effective population size of the species has been shown to have been in the hundreds of thousands range.
Oh, and the Carolina parakeet was bred in zoos, even in Europe. Unfortunately they weren't that consistent back then yet.
The reason why researchers go for those relatively boring species is their boringness: there are closely related species from which you can borrow cells (and eggs) which you than modify. There is no such thing for a huge (another cost factor) animal with no close relative left.
The idea of bringing something old world into current is compelling, definitely morbid in some sense, and primal.
Yeah, not a good mix, but what can I say? This is a want to see happen. I am likely to regret it, but maybe, just maybe it could be a good thing. Somehow.