There is no way to tell if there's any "casual" guild will be returning to WOW TBC Gold Season of Mastery. The only thing I'm interested about is how many other players like me would be interested in this.
It is difficult to desire to play SoM as it will most likely have the players base that P servers do not. But it's also a cash grab and weightless experience. There's nothing new, other to tweaking some values to make the game harder. It can make the game less fun. The insufficiency of loot could be irritating and it could be frustrating with the timeline. It's a fantastic idea, but no world buffs are good! People were actually calling for it before classic even cheap Burning Crusade Classic Gold became being considered a thing. This is the time!