Id like to see the return of the several species and subspecies of bison. Like mammoths , there were many species of bison that were boreal browsers to plains grazers and even several species in the Asiatic foreland (steppe buison, Long horn bison 'boreal" etc) It would be nice to reinsert some genetic variability into the genus and link ll the species together on a map so that these megafauna could be part of our present worldwide biological zones.
Also, Id like to see passenger pigeons and the large flightless ducks, theMoa nalo
of the Pacific islands. Some of these were reportedly large goose -like birds that were divers and lived mostly on water but came to land to lay eggs and raise their young. large ratites like the Moa or Aepyornis maximus
of New Zealand and MAdagascar were hunted to extinction in less than 10 human generations. Itd be neat to have them back , and this time we would try to keep em from being barbecued .
Very large carnivores like sabre toothed cats, dire wolves or short faced bears, Im not sure we should try unless we have several large habitats set aside.
Irish ELk would be dead on arrival IMHO. These things would be getting their antlers stuck in snow fencing and barbed wire. The thickets and hedgerow of present day Western Europe would just be difficult for these things to traverse. Wed have to saw their antkers off for a few generations until the elks got a sense of how big their racks are.