I also think that if they were to make a subsequent movie with Blake as Batman, it should continue in Nolan's relatively grounded universe - a move into a world with superpowers and aliens would be a very dramatic shift.
Not necessarily. In the recent interview, Snyder said that producer Nolan insisted on a grounded and realistic approach to Superman, with some scientific justification for things (or so it sounded to me). And let's face it, there's plenty of outright impossibility in the Nolan Bat-films, from the inane microwave weapon in Begins
to Batman's shattered-bullet forensics and the cable impossibly strong enough to flip over a truck in TDK to Bruce's magic recovery from a broken spine and the fusion overload that can be timed to the second in TDKR. Despite their pretensions of realism, they are fantasy movies. It's just that the impossibilities are presented with the trappings of realism, and in the context of an at least somewhat grounded and naturalistic character story, so we're willing to suspend disbelief.
The JLA movie seems like a perfect opportunity to relaunch Batman in a different continuity than Nolan's. And despite Nolan's connection to MOS, I'm very sceptical of the claim that Blake will cameo in that movie - it seems at odds with Nolan's insistence that he wasn't working towards Supes and Bats sharing an onscreen universe.
True, but there have been plenty of instances where a creator has had one intention at the start but has then been persuaded by his employers to change those intentions when enough money was at stake. (E.g. Conan Doyle killing off Sherlock Holmes in "The Final Problem," which at the time he intended to be permanent and irrevocable.) Gordon-Leavitt's a big star now after TDKR and Looper
. The studio's desire to capitalize on his growing fame may have outweighed Nolan's desire to keep the universes separate. It's not as if Nolan is the only decision-maker involved here. Ultimately it's Warner Bros. and DC Entertainment that he answers to.
That said, of course, a rumor is just a rumor, and "sources" who don't give their names shouldn't be trusted, at least until there's independent and official confirmation. I'm just saying it's possible.