Well, upon first seeing the picture of this 'Mythic thing' for Halo 3 it seemed like something that was going to be pretty exciting but after doing a little research I'm pretty confident it's nothing to get too worked up over.  Judging that the screen shows only multiplayer menu options, I'm putting my money on it being a showcase for a new map pack (The Mythic Map Pack, perhaps?) coming out, a la the multiplayer beta released to the public.   Also,  Bungie's Brian Jarrard said that the speculation is being "blown out of preportion" and it is "really not that exciting".

I think it'll be a downloadable expansion.  Maybe a couple of new levels, and a boss fight.

I'd agree with you.