This is kind of making the rounds lately, although I've yet to see an official word from Lucasfilm (Same with the Obi-Wan film which has been rumored for quite a while but no official announcement). Reportedly James Mangold is attached, which seems like a decent idea-the guy did, after all, The Wolverine and Logan were both, in a sense, "ronin" stories, and Fett is clearly inspired by Clint Eastwood's man with no name, which is of course based on the Kurosawa Yojimbo film....sort of 'full circle', although I'm not sure Mangold would necessarily take the same approach.
Kind of curious though how the film would go, though. Would Temura Morrison, Bodie Taylor or Daniel Logan be brought aboard, or somebody with a reasonable resemblance to them? I'm pretty sure Fett wouldn't spend his entire solo film wearing his helmet. Would the film reveal Boba somehow survived ROTJ, like in the original EU, or take place earlier on? Or perhaps have the armor be passed on to someone else, somehow?