Honestly I'm being kinda facetious about a full fledged reunion but... I think that it was a mistake for Al and Miles and-later on-Brian and Kelly to forget there was an extra Luthor heir out there. We go through all that drama at the end of the series about Lex and Tess and it's like Lionel had no other kids. I find it hard to believe that Lionel would just write off such a devious seed as Lucas. I mean he basically is the Luthor ideal and he's just forgotten.
I think Lucas would make a neat speed bump in Lex's arc this season. He's busy trying to figure out his life again and then BOOM there's a baby brother who's more
than willing to take his place as Metroplis's magnificent bastard!
Heck even if they skip it this season it might make a nice arc for Season 12.