Nobody is quite sure what's really going on with the planned Batman solo movie. While the project does seem to be moving forward, though quite slowly, it's far from confirmed that the current DC film universe Batman, Ben Affleck, will actually be in it. Director Matt Reeves isn't helping the situation. While he confirms that the script, which he expects to have a completed draft soon, is connected to the existing film universe, he's not confirming Ben Affleck will be in it. According to Reeves...
There are ways in which all of this connects to DC, to the DC universe as well. We're one piece of many pieces so I don't want to comment on that except to say that I'm focused very specifically on this aspect of the DC world.
The fact that the Batman solo movie does, in fact, connect to the DC universe is important because we know that not all movies that Warner Bros. is currently working on actually do. There's a Joker movie coming that won't be related to the existing version of the character played by Jared Leto, and it was always possible the Batman movie could end up working like that. However, it seems from what Matt Reeves recently said at a Television Critics Association panel (via Slashfilm) this movie will absolutely be part of that universe.
But with that being the case, you'd expect Ben Affleck to be in it as he is the current DC universe Batman. Matt Reeves does say that he's still speaking with Affleck, but he's clearly stopping short of making a statement on the man's participation as an actor one way or another. Affleck himself has suggested that his days as Batman might be numbered, and it's not like Reeves saying that Affleck would be reprising his role would actually be news WB would want to keep under wraps, so it certainly does sound like there's a possibility that somebody else could play Batman here.
Of course, both things could still be true. Wonder Woman was a movie that took place years before Justice League, it just happened to star the same actress because the character is immortal. While Reeves has stated that the new film is not based on the popular Year One comic, it doesn't mean we won't see some sort of flashback story that sees another actor play a younger version of Affleck's Batman.
Matt Reeves says the new Batman movie could start shooting as early as the spring or early summer of next year, which means these questions will need to have definitive answers sooner rather than later. It looks like the Batman movie is clearly going to happen, but who will be the Batman is still an open question.