We probably can't expect a hidden subclass for all classes, but I wouldn't say no to a DLC area where it's possible to unlock them through one combined quest. Essentially another story area with the subclasses as a bonus.
As is, I would only want more in the default game for currently underpowered classes. I'm not a big fan of Larian neglecting some classes while playing favourites with others. This could level the playing field a little. Gloomstalker and Hexblade are especially thematically appropiate. Wizard could also use something to keep up with the 4 fireball/turn sorcerer.
As far as I can tell, Twilight cleric:
-- has 300 feet darkvision, which can be shared through using an action (one hour, once per long rest)
-- can boost initiative pre combat for one creature (1st level)
-- gets a moving sphere giving 1d6 bonus hitpoints /or/ ending Status effects like charmed or frightened (If you start your turn in it)
-- flight at level 6 (this is a BONUS ACTION, useable equal to your proficiency bonus. Restores with long rest)
-- Divine Strike at 8th level (+1d8 radiant damage on weapon attacks, 2d8 at level 14)
-- Twilight shroud (level 17) half cover at demand within the "Twilight sanctuary" sphere you previously unlocked
I would personally prefer the Shadow domain as listed here
https://www.dandwiki.com/wiki/Shadow_Domain_(5e_Subclass)
