The rewards are going to have to be a lot more enticing than pets, mounts, transmog items, etc, to get enough people to want to do scaled content.
If the only reward was cosmetic, the feature would die due to not being able to get 25 people together, let alone 40.
They could add LFR transmog sets we don't have access to anymore as rewards for an achievement by finishing the raid, or possibly have their tokens drop with a random chance at each boss.
They could also approach it in a different way to just redoing the content, they could be parallel worlds/timelines where we failed and we're going in to save the day as we already defeated these foes in our timeline. And as these will be slightly different we could possibly also get some nifty costume sets from past NPCs. Just a thought really.
There needs to be a faction towards which, in addition to reputation, you can earn spendable points (like Valor and Justice) by raiding old, upscaled content. The faction then would have an NPC like the Upgrade Vendor who could "zap" the old items somewhere close to current tier items; maybe the cost could be adjustable so that the more points you put give the NPC, the closer to current raid tier the items would become. These these "old content raid points" could be used to purchase all kinds of things in addition to the upgrade process, such as mounts and extra cosmetic items.
In essence, you'd need to grind the old raids a bit and get the drop as well (which would drop as the old low level versions) to get a current "Timewarped" version of the item. Without upgrading, you could still use them for transmog, of course.
The advantage of this solution is that they wouldn't have to go and tinker with the current loot tables at all; just add a new point currency that drops only from upscaled raid bosses and then add a vendor or few that dole out the rewards for those points. Would also make adding/changing rewards a lot easier in the future, as opposed to changing the loot tables again.
Honestly, cosmetic rewards, mounts and recolors of existing gear would be great.
But people would likely run them, as long as bosses drop VP. Once you're geared enough and just run HC dungeons for Valor Points instead of gear upgrades, it gets so incredibly boring to run the same ones over and over. Having the option to run all the old dungeons, whether those dungeons are scaled up, or you are scaled down, would an incredible relief, as a way of gaining VP in a different setting.
So everything should become a quest for gear? No thanks.
It would have exclusive vanity items, transmogrification gear and mounts. Not the same as the type of stuff you could get for running the normal version solo.
There was an example in a few posts before I made.
Things like, for AQ40: A red/green/yellow (black and blue already used) qiraji mount that you can use outside, or maybe a wasp mount. Updated Dark Edge of Insanity transmog item? (there is an updated version of that weapon in the files)
And for things like Blackwing Lair and beyond, they could have the remaining unused (1 or 2 ) T2, T3, T4 etc colorations and make them drop from this mode. Things like red Battlegear of Wrath (T2), the Blue Ymirjar Lord's Plate (T10, the color many people wanted to have) from ICC.
And other exclusive rewards.
For some time, I wanted to add that the mounts that had a 100% drop before but were lowered after the expansion (Mimiron's Head, Invincible, Pureblood Firehawk, etc) would have a 100% drop rate too, but I don't think many here would like that idea.
Even could do it for Naxxramas and make the Naxxramas drakes drop there, oh .. and the original T3. (that is now only available from BMAH)
I still think it's a good idea, but probably, a lot of people would pounce on me.
Why not? Let's face it, gear is a much greater motivator for something than transmog and mounts - think about it this way, how many people have at least 20 mounts? Do these people rotate out and use different mounts on different days? Probably not lol. I got 168 and I use my gyrocopter like 99% of the time lol.
May make things interesting for a lot of players if they put more ways to advance your character - and I don't mean cosmetic stuff. I can tell you right now that while some were interested in challenge modes, including myself, I never really had an opportunity to do them just because they provided no sort of advancement.