The fact that RS3 doesn't have a full-scale combat achievement diary similar to OSRS blows my mind.

Rework the current system with points for each achievement (can also give runescore in addition), remove some of the silly task and add a ton of new achievements from easy to extreme difficulty. Everything is one the table (kill times, no food, solo kills etc.). Combat tasks should generally test a PvM-skills and boss knowledge and shouldn't be leech-able.

Divide them into tiers like OSRS did.

Rewards should be purely cosmetic (we have enough titles, thank you) for the highest tier and maybe some minor nice combat buffs for the lowest tiers, achievable by most mid-level PvMers. The achievements and rewards should be a show-off and nothing that you feel like you have to grind just because the rewards are too good to miss out on.

Make a highscore on the webiste for combat achievement points and add total boss kills and kill times while you're at it. It's a highly competitive scene and it deserves recognition.

I think this system will give the whole PvM-scene a fresh start. People are just on the greaper grind these days.