Could definitely be R.E.B.E.L.S., but you can get the Legion involved without specifically invoking time travel--DC has often had big crossovers have repercussions 1000 years later (like having the 851st century Legion show up when everybody else was dealing with the 841st during 1 Million, or by having a few surviving Manhunters show up during Millenium).
Say Brainy finds a long-lost but functional Black Lantern ring--never mind how it's functional, it just is. I don't know about the newest incarnation, but plenty of previous versions have certainly had dead folks they wanted back. Even if that's not how the rings are supposed to work, just put it in Brainy's hands for fun.