Plot reminds me slightly of an issue...
Does anyone else remember the short 2001 run of Suicide Squad? There was a two-part issue (#8/#9) where an island of wizards appeared and the squad was called in (of course in this one Waller was smart enough to pick a formidable team to deal with the threat).
The issue ended with the magic user (Blackstarr) of the squad defecting and basically becoming the leader of the island - the island warped away.
A little tweaking and it reminds me of the movie, a severely powerful magic using member of the squad turning against the other members of the squad and becoming a threat (although in the comic it was implied she'd be back to wreak havoc rather than actually wreak havoc... unfortunately the series ended before that happened, and not sure if it was referenced again later).
But it would've been so much more interesting!!!