Personally, I've never really been a fan of the character, because
I always found him a bit similar to Superman -which kind of makes
one of them redundant on the JL, and since MM can do a whole bunch
of stuff Supes can't , the it's Supes who's redundant......and Supes is
never redundant.
However, when Supes wasn't really involved with the JL, back in the
Giffen days, I thought the character really moved streets ahead.
Having said that I reckon a JL film would benefit from having MM.
But, the difficulty you have with characters like Shazam and MM is that
to the general audience, (as I said) they're pretty similar to Superman.
Sure, comic book fans know the difference, but to work on the big screen
there should be significant points of difference - I suppose that if shape shifting and telepathy were shown as his primary powers that would work well.
As for Flash, GL, WW, Bats and Cyborg, well they're all radically different than Supes, so having them makes sense.
As for MM, why not ? Although I think casting would be all the difference.
The Rock....no, not cerebral enough. Provided it was done well, and with the right actor, why not indeed ?