The title say it all.
Edit:
How about a bonus boss of the expansion being the only 40 person boss. Of course the carrot would be legendary gear or something like that uniqueness. Also the boss would be available to only the server so no lfr. Guilds would have to team up like the old days (ermahgerd he's talking about socializing with new people).
Here's my idea it would be a separate raid entirely and it's just 1 boss like an onyxia type raid. The raid would have difficult trash packs with unique abilities like the TK trash that required cc and off tanking and the final boss fight would require vashj like coordination with a little muru thrown in there.
All the players could see all the content in the current lfr system and kill all bosses of the expansion. They would get everything except the bonus boss.
Remember I am advocating 1 and only 1 raid boss to be a 40 man boss.