Idea 1, Account wide Achievements, Collections and Seasonal Rewards:
A big issue with current classic is the constant feeling that it will come to an end, or the server will die and you will need to make a new character, maybe your just playing the latest Season of Mastery. The issue remains that eventually it will come to end, so why bother farming for that mount? Why spend ages getting that achievement?
A account wide achievement and collection system could bring more longevity to the game.
Get Invincible in WOTLK? Be able to use that as a mount in a TBC Season Mastery!
Complete all seasonal SoM achievements, get a cool Cape transmog you can use anywhere on any character!
Give players a feeling that hard work on one character has long term effects and that SoM players have some seasonal achievements to aim towards with rewards!
Idea 2, OSRS style Classic +:
I know this has been said before, but I think this really is the best direction to go.
Just release vanilla classic again, and then start building on it as its own game, it doesn’t matter is everything is canon or not. Even give us some “classic versions” of other expansions content or remake some content pulled from the game like Karazan Crypts!
Let players vote in-game about new updates and features they want to see, like new professions, a new area or raid. Don’t worry about making entire new expansion for it, some times a new raid with new loot can be enough, just a new zone with some cool story quests about some lore the main game never went that deep into with a few rewards. Make more crafting options to drive a player driven economy!
Conclusion:
I think so many classic players just want a feeling that there time in classic isn’t going to waste and that there will be a future for it. Back in 2008 so many of us would spend time in game just trying to get old achievements, getting groups to farm a mount, flying around to get the Explorer’s Tabard and doing every quest in the game for Lore Master!
and we did this because never thought about the game ending.
Anything that help bring that feeling back is a step in the right direction.