I'm not the biggest fan of McGregor, but I'd be interested to to see what he can do with another shot at Kenobi. I think the cast of the Prequels weren't given the best shot at being able to give their best performances. Mainly due to having to act with little to no actual environment or other on-screen characters. The CG heavy nature of the Prequels' production would have been hard on most seasoned actors I think.
Given how McGregor is already well established as Kenobi for chunk of the fanbase, mainly younger prequel era kids I would suggest, I think he's a safer bet than the recasting for Solo. Although I thought Alden did a pretty good job of delivering the essence of a cocky younger Han and I was one of the sceptical ones in the lead up to the film's release.
With a Fett movie Disney could potentially make a mistake in unravelling a lot of the mystique which makes Fett such a fan favourite. If handled properly an underworld, bounty hunter centric movie with a Fett cameo could be really good in my opinion.
The most important thing Disney has to do when meddling with beloved characters as old as the OT is respecting the source material. For characters like Solo, Kenobi, Luke etc their characters are very well established so any major deviation from lore needs to be very thoughtfully considered. I think there's also an element of just because you can, doesn't mean you should. One of the reason I liked Rogue One so much was because it centred around a bunch of new characters. I bloody loved the Vader cameos and I realise they were done purely to tickle the balls the the OT fans, but Vader and Tarkin acted very much in the vein of that which we're used to in the OT (not including Vader's sabre slashing...which was a particularly badass new addition). I'd love to see more Star Wars Stories like this; new characters and new worlds.
Chris