Rant warning: 
I hate this idea that DC, and many fans foment that Captain Marvel/Shazam needs  Superman or worse Batman to appear and validate him, as if he's awaiting acknowledgment and  their approval.
This is how DC (and fans) have been  undermining  the character  since they acquired him, and why most reviewing the trailer start with "I don't really know or care about this character, but...". 
I'm fine  with the movie referencing  the DC heroes and establishes it's the same U, that's great! 
Although if the fans get their way, Superman and Batman and all  will soon show up to say they approve of Billy/SHAZAM!,  subjectively  relegating  Captain  Marvel/Shazam!  as subservient to them. While the fans applaud and squee look Superman and Batman approves! 

Casting and relegated him to second-string,  permanently in their shadow. It's the fans who feed into that.  
The  character  existed for decades as the definitive champion of his own universe.
The after credit reveal shouldn't be Superman, if anyone should be Teth Adam.
And no SHAZAM! shouldn't ever need Superman and Batman to help defeat, or find the clues to defeat Adam.
That is Billy's  journey.
 
 As far as eventually Joining JL, again this myopic vision that you don't count until you're in JL, has also relegated  SHAZAM! into afterthought second tier joiner status. Which he wasn't.
If anything he  was,  and should be the champion of his own team.
With some updates: 
A contemporary Spy Smasher
Ms. Scarlet (fem update above)
Ibis existed before Dr. Strange. 
Bulletman & Bulletgirl/The Bulleteers (also updated costumes above (more Rocketeer-like))
This attitude of how does SHAZAM fit into the DCU, needs to be dropped, and the focus should be how to develop SHAZAM's U, where he is the champion. And then the DCU characters can come play.