This came from a forum thread.
While scaling legendaries would work to my advantage, I think it would be counter-productive.
I personally saw that people were burnt out on MC. I had a hard time getting fun runs and ultimately learned to solo the bosses (though both bindings dropped while I was in groups). If MC became a requirement, even just in the sense of being an alternate progression path, I can see a lot of people feeling forced into it. While I think MC is still fun, most people seem to disagree and I think it would be terrible to 'force' people into it again.
Scaling would tend to dilute the legendary-ness of legendaries. They would become far more common and would be little more than another piece of gear to collect. As they are now they are rare and for the most part, useless except for appearances. As strange as it sounds, uselessness is one of the most special things in this game where people are so quick to jump on the easiest, best gear as soon as they can.
Forcing people into old content and turning legendaries into another piece of gear to farm would be terrible. It is unfortunate that people rarely do the old raids, but it's difficult to add incentives for old raids without create a sense of them being mandatory.
I wrote the previous part of this a couple hours ago. Since then I've warmed a bit to the idea. However the balance issues remain. If it scaled like an heirloom item then it would be outdated anyway, since heirloom items are slightly worse than equivalent blues. The scaling could be bumped up, but then it's starting to give gear rewards just for leveling up while the heirloom items are intended to make leveling easier rather than make leveling a replacement for instances and quests in terms of gear acquisition.
I thought of other scaling methods. One was to make them 10-15 ilevels behind the current BiS. However this means that they would get stronger just because a new raid came out and that makes no sense. They could scale with anticipated BiS for the expansion and then be further behind, but this would mean making them exceptionally powerful early on, and then Blizzard would be forced into a set gear range or else the weapon would end up either too powerful or too weak compared to the original intent.
Finally I settled on scaling with a current chosen weapon. This would use an idea I've seen floating around the forums: stat copying. Take a current weapon and map its stats onto the legendary, replacing current stats with the exception of any strange abilities such as the TF proc, Sulfuras proc, Atiesh on-use (and group buff), or the set bonus on warglaives (though they're not really legendary). This would allow players to use pre-existingg legendaries without gimping stats without giving the perception that legendaries are a requirement. The legendary itself would be effectively an aesthetic item rather than another stat carrot to chase.
Unfortunately that idea would probably be very hard to implement. While I'd love to see it, in the grand scheme of things its probably not worth the effort to implement.
My goal is to find a middle ground. I would like to see legendaries still used. However I don't want to see people forced into doing MC again. By stats I think legendaries should not be special, it should be by the way they are acquired (damn you, warglaives), appearance, and the special effects on them which cannot be found elsewhere. Perhaps the biggest split I have with people is that they think that legendary means eternally best in slight while I think they should be eternally cool and perhaps useful, but not at the expense of new gear.
I suppose I should apologize for the many posts about legendary stuff recently. I probably sound like I'm bragging. I assure you, only two of the posts are for bragging purposes (okay fine, that's still two too many). The rest are just ideas that came up because I was playing with my TF and thought of them. Or this one which was inspired by the forums as sometimes happens.
[edit: added sometime later, I don't know what time this site uses]
My latest thought would be to imitate gemming. Add a weapon socket to legendaries which would fit anything of that weapon size or below: so a 2h can fit anything, a 1h can only fit other 1h weapons; so no putting a betrayer of humanity in TF. Rather than complex stat copying, just let me stick my Red Sword of Courage into my Thunderfury and gain the stats, with the exception of weapon DPS which it would disable and replace. This would cause the socketed legendary to have slightly higher stats than the socket weapon (ooh, watch out of my extra agility and resistances!), but I think that is almost insignificant.