I'd personally love challenge mode black temple, ulduar, tempest keep, naxxramas, icecrown citadel, molten core.
Make them award a pet for bronze medal, title for silver and a mount for gold. Imagine a token you get for completing all on gold, and you can redeem it for any rare drop mount from vanilla-wolk raid. (invincible, mimirons head, ashes of alar)
Your ilvl would be reduced to the raids equivalent. (264 ICC, 226 Ulduar, 200 Naxxramas, 89 Black temple and 60 molten core) Don't quote me on the last two though. Bronze medal would be kill everything with 1-2 wipes on each boss, silver would be maybe 1-2 wipe on one or two bosses and gold would be a flawless clear in a certain time, you get the idea.
Just my idea, idk what you think.