Possibly a controversial choice. The Xindi were in fact six linked species that became one of the main antagonists of Star Trek: Enterprise. Season three became a single story arc spanning 26 episodes, with the Enterprise entering the expanse following an attack on Earth by Xindi. It was ultimately part of the convoluted temporal war arc that ran through the entire series (the Xindi launches a pre-emptive attack on the Federation after being manipulated into believe they would destroy the Xindi species in the future), but they served up some much needed excitement after the rather disappointing first two seasons. With the new Star Trek films set a century later it would be interesting to link the prequel series together with this one by showing us where the Xindi are now. Due to the different facets of their culture they could be both villains and allies and that alone gives them a wealth of opportunities for a return to Star Trek. But not only that, each Xindi species offers the audience an opportunity for a variety of stories and settings. We have aquatic Xindi (complete with aquatic warships); Xindi-Aroboreals, the scientists of Xindi culture; the flying Xindi (though apparently they may already be wiped out), insect-like flying Xindi; primate-Xindi and finally the reptilian Xindi, who proved to the most aggressive of the six species. And considering their love of eugenics and the open plot surrounding Khan and his followers at the end of Star Trek Into Darkness, this might be the perfect chance for them to remerge.