Glad you like the ideas :) Like I said, with these achievements, there are endless possibilities when it comes to unlocking new badges, it's just a matter of thinking of the different tasks if it does get approved.
This is a really good idea, it'd motivate alot of people to grind these certain tasks, if it means that they get a displayable reward in return.
I made a few examples of what some of the awards could be. Ofcourse it's not set in stone, however it's always nice to visualize -
Award Examples
Slayer Helmet (1k KC on multiple Bosses)
"Unforgiven | Given to those with a kc of over 1000 on 3 different bosses |"
Callisto (200kc in wilderness bosses)
"Unbeatable | Given to those with a kc of over 200 on 3 different wilderness bosses |"
Raids 2 Chest (100kc in Theatre of Blood (100kc in Chambers of Xeric)
"Determined | Achieved 100 kc in the theatre of blood |"
Raids 1 Lootbox (100kc in Chambers of Xeric)
"Persistent | Achieved 100 kc in the Chambers of Xeric |"
There are lots of different awards that could come out of this suggestion, however with the main ones that you have suggested, I believe this would be a great addition to the community as it will add them extra challenges that end games players need.