Always been totally against p2w, but since thats the way they went with the revenue model, at least do p2w correctly.
There should be a panel that can be toggled off for those who don't want it, which shows monster kills by the number and shows you the potential reward for meeting the threshold provided you own the manual that monster is in. It should also be intuitive for each zone. Now every zone is a slayer zone.
You should get discounts on some merchants or be an alternative method for being able to buy better items on favor merchants. For example, the silver flame should roll the red carpet out for me if I've killed every type of vampire and hit certain number thresholds on the more common ones.
They could also do monster hunter titles. Getting a rare title to display that required owning multiple manuals and doing a wide variety of content, everything from raids and slayer zones to quests, would have some bragging rights behind it. Even link some to HC. Someone could unlock a HC Dragon Hunter title for their entire account if they killed every dragon in the game at least once on a HC server.
Start designing more cosmetic items for rewards. If I kill one of every mob that has DR broken by silver I should be able to choose a silvered cosmetic weapon or silvery arrows, both of which leave a silvery trail when swung or shot on their way to the target. Maybe a silvery orb for casters, or a staff skin.
There can also be a link to damage types. Not only did I kill every single type of vampire, I used radiant damage to do so, so now I have this cosmetic emblazened armor of the sun, or a shield with the same logo.
There are waaaaaaaaaaaaaaaay too many unexplored options here. It feels like it was someones downtime project that somehow made it into the game but doesn't get updated enough to matter for the majority of the players.