Did you play RuneScape in the past? Yes, I have an Completionist Cape on my account in RuneScape. This means that I have completed the game OSRS Accounts. What? Did you actually kill the toughest boss? No, I actually haven't killed all the bosses from RuneScape and how do you know you have done it?
It is clear that combat is a key element in RuneScape. The official RuneScape manual I received from the 2006 scholastic fair states this. Bosses play a significant part in combat, and it's only right to all of them fight the Completionist Cape and take on the other characters. Not to say I'm a huge fan of it myself, Yakamaru will be a great dog!
The problem I have with this is the team bosses. You can't assure yourself that you'll be able to find a team. Being capable of being "complete" is something that can be guaranteed and doesn't rely upon other players. As for abilities, that would have been viable prior to the release of raids.
But now there are tradeable abilities that can be traded, and having them all as a requirement is similar to having all weapons in RuneScape as a requirement. While it might have been possible in the past, it was dropped from two bosses Buy RuneScape 2007 Gold. Additionally, I've added more information to my previous post than your last post if you didn't notice and want further reading / discussion.