Weird.
Why would Shazam be the follow up when he's not quite as known as WW, green lantern or the flash. Same goes for sandman.
Also, is it necessary for shazam to be a part of the JL on film? He's almost he same guy as superman. This is the reason War Machine doesn't make an appearance in the avengers (just a 2nd iron man) and gambit didn't make an appearance in the first 3 x-men films-- gambit's rogue personality s too similar to wolverines.
I think teaming the flash with green lantern is interesting though.
If you're kick starting the franchise with the two biggest names (bat/supes) with appearances from other heroes, then it would be a steep decline to focus on just one less popular hero by themselves.
So you have a flash/lantern team up then maybe a Wonder Woman/aquaman team up. Then reunite them all for justice league.
Also it keeps you from having the same question that pops up in every new marvel movie... "Captain America is wanted by everyone in SHIELD, why doesn't he call for some help from stark, or banner?"
At least some of the team stays assembled.
... Yeah it's 8:00 on a Friday. I'm cool.