Even if the proposed challenging content YoshiP has suggested is not a third raid tier, I still feel it's important for the reward for this challenge content to be vanity-only. The idea is to have a strong challenge for people looking for this challenge - these players don't need any carrot-on-the-stick reward scheme though a vanity gear/title/minion to show off "We've done the content!" would be very nice.

I don't know much about it but I feel the master trial recently released in XI is a good example. Literally just a whole fight stupidly tough boss mob(s) for a level 1 glamour weapon.

INB4 "EEEEEEWWW FFXI"