On that point Classic has done WOW TBC Gold a great job more than any Classic + edition could. What you're asking for is an old game of 15 years to be inventive and introduce new well everything, they did, its called expansions and while there are some that don't, it's basically Classic +.
Furthermore i severely doubt any "casual" guild is going back to Season of Mastery, only part bout this whole thread is my curiosity about how many other like-minded players really want to do this.
It's difficult to convince yourself to play SoM in one way, but on the other hand it's probably going to have the players base servers that don't. On the other hand, it's an easy cash-grab and a sluggish experience. You're able to tweak some parameters to make it more difficult. There are no new features. In a way, it makes it less enjoyable in my opinion, the cheap Burning Crusade Classic Gold scarcity of loot is a real problem and with this timeline it can be extremely frustrating if you aren't lucky enough. World buffs are great and all, but there were people calling for this prior to the time the term "classic" was even a thing! !