As a player i feel like i'd like to put my suggestions out there.
Firstly i think there should be a High levelled boss for either Solo, or Team.
~ boss would drop High level items such as Twisted bow, Ancestrial Peices, elder Maul, Dragon claws, Dragon harpoon, Augury prayer scroll, Rigour prayer scroll, Etc.
This could be a Raids boss with Raids items, but the drop rate being not as easy as people would expect, so alot of kills would need to be grinded to keep a high value for the item.
200m capes could maybe be a choice for players hard+ as normal is ridiculously easy to get any 200m, hard on the other hand has a little bit of a test to it, then players could show off the nolifing skills they have.
PERSONALLY i feel like the 500 player kills achievement is absolutely ridiculous due to me being a none pvp player, and to achieve Completionist cape it will physically be impossible for me to do it in a Legit way with all the Killstreaks required.
Maybe change it around abit to give other Skiller/Pvm/Pkers a wide set of choices, even small achievements like:
Woodcutt 500 magic logs
Woodcut 500 yew logs
Mine 250 Runite ore
Mine 100 Adamantite ore
Smith 100 rune platebodys
Smith 100 adamant platebodys
Make 200 Supercombat potions
Complete 100 Wilderness agility course laps
Catch 500 Red Chinchompas.
Burn 500 Magic logs.
The list could go on.
Personally i think the achievement list isnt very good for comp achievers.
The 200 hours one could just be afked for a few days and wouldnt be a problem to complete, Maybe a AFK timer to log a player out after the AFK Patrol guard has put them into jail, give them 1 minute to talk to him and log them out.
Any more suggestions ill add to this post. or PM me on forums and i'll add it for you.
Thanks.