It’s worth noting that EW call the film “a Boba Fett/bounty hunter movie”, meaning that while everyone’s favourite Mandalorian would have featured, the story focus would have likely gone wider, raking in a wider team dynamic or exploring the culture of galactic bounty hunting. This would actually line up with a rumour from around the time of Trank’s departure that Boba, Cad Bane and others would appear in Rogue One; it now seems there was some truth to that – the reports were just discussing the wrong movie.
The big question is when this would have been set. So far, the announced Star Wars Story films are all based just before the era of the original trilogy, and as canonically Boba died in the maw of the sarlacc in Return of the Jedi, this would have presumably also been around the same time. There is of course the possibility the film would have lifted from the Expanded Universe and have him escape, but otherwise it’s a very tight period, which is a potentially worrying sign for a franchise that is supposed to be expanding. Of course, as the film never even got its test footage out, it’s not worth drawing too many conclusions.
What we do know is that Trank’s departure halted all development on the project – it was originally set for Han Solo‘s 2018 slot, but was pushed back and now lurks on the extremities of the slate. Whether it ever gets made is up in the air, although you can bet that now word of a teaser reel is out fans will be clamouring for a look at what might have been.