There will be no ''Legendary Weapon''
Last part of Wrathion MoP questline will award a legendary gem with role speciffic proc effect, akin to those present on past expansions legendary weapons.
Weapons dropping in SOO will be able to get socketed with said gem, aswell as with sha touched gem and extra prismatic socket.
That way, you will get powerfull weapon with +500 stat, extra socket and legendary proc, but weapon itself will be of varying Ilvl depending on it's origin (LFR/FLEX/NORM/HC).
That is the only way they can pull it off, for a number of reasons:
1. Obviously they won't give BiS weapons from running LFR.
2. Having BiS weapon from questchain removes large part of ''upgrade anticipation'', since we all mostly look forward to getting weapon/trinkets/setbonuses.
3. Making weapon non-BiS to circumvent 1. and 2. makes it just a vanity item, and nothing truly ''legendary''