Here’s my feedback on the tome slot requirements after having unlocked the tome slots on two of my classes.
Most of the time slot requirements are actually not that bad. Things like prof levels, killing mobs, completing loot runs, completing raids and even warring are requirements I think are great as the requirements you’re grinding for actually benefit you and make use of the tomes you’re trying to unlock.
The requirements I do have a problem with are the ones where you have to engage in non-repeatable and preferably one time style content like quests and discoveries. For your first play through, you’ve likely done most if not all of the available quests and maybe a good chunk of secret discoveries. Though an argument could be made for secret discoveries being a requirement for a tome slot, I don’t think quests (especially 50 unspecific ones) should be a requirement for a tome slot after your first play through on a class.
As for an alternative, here’s one I thought up:
A higher unspecific quest requirement (current quest requirement per class is 50) on one class to unlock that tome slot for your whole account as opposed to 50 per class. This would encourage the player to experience a very good portion of the quest-line while also cutting the total quests required and dull repetitiveness for a more experienced player who intends to fully utilize all 5 (10 if you count cosmetic) classes.
The same principle could be applied to the secret discoveries (currently requires 25 per class) as well.