In the retroboot, IIRC, there was a poster of Cosmic Boy from his time as a young sports celeb on Braal in Lightning Lad's quarters and I kinda dig the notion of Rokk having been a bit of a celebrity as a teen (and now being even more of one as a Legionnaire). It creates a fun dynamic if the three founders come from very different experiences. Child sports star. Farm boy with a messed up family situation (thanks to Mekt). Studious wannabe cop.
That all said, while I could stand some real exploration of these older characters, I'd even more enjoy a story dealing less with the founders + Brainy, and more with younger and less-developed and diverse Legionnaires, like Tyroc, Myg, Glorith or Dragonwing or those from other continuities like Kid Quantum II, Dragonmage and Gazelle. I'd be fine with the 'old guard' being some of the 'newest' of the classic Legionnaires, like Dawnstar and Timber Wolf, and much of the first fifteen or so were retired.