Dick Grayson is arguably the closest one can get to being a Trinity character without actually being a Trinity character.
DCOIE was designed to build up Nightwing and he did perform well in the story, BUT the ending had Michael Holt narrating the results with various characters with a group of heroes. Now, if Nightwing was going to be a major player, then having him handle the assessment was a much better story finish for him than going off with Batman.
I'd like to see Grayson play a huge DCU role. He's not Batman level, but pretty damn close. He also has a charismatic and inspirational leadership talent which is what makes Superman more than just another flying brick. Although he comes up short against the World's Finest duo, his possession of both of those aspects is something neither Bruce nor Clark have.
If Dick does get a JL type team, it would need an innovative and explosive pull like the start of the DeMatteis/Giffen/Maguire run. IMHO, of course.