Rewards for scaled up runs could be reputation gains, lesser charms, or the new version of what they will be in future content. This would make a great substitution for dailies.
Justice points and a wider variety of items available for JP like crafting mats, hard to track down mounts/pets, etc. While I don't necassarilybuy into the notion that players are "forced" to do all available relevant content, rehashing old raids runs the risk of diminishing new content. Making it alternative rewards, like those I mentioned, serves to make them more attractive without making them compulsory.
Setting a reputation....for instance, Netherwing (to get the drakes) or some such. That said, they may have over played their hand with making mounts available across characters, and as such, make this less attractive.
Perhaps quests for bonus JP? I still do the one out of Dalaran most weeks for kicks and grins since I'm likely doing Naxx or Ulduar solo any way. That set up may work.
As a (fairly random) different way they could go with the scaleable old content:
I'd quite like to see a new style of challenge mode for old content, with item level the limiting factor rather than time - ie how far can you scale back your char and still complete an old dungeon with less than x wipes - with bronze scaling somewhere close to the item level you would have been about level appropriate. JP / VP / rep seems as good a way as any to make it optional (perhaps a new 'Keepers of Time' sub-reputation, with some novelty items). It could even be compatible with LFG, if using it automatically scaled it just to bronze (ie level appropriate).
I'm all for more / quicker content over new character models (unless the new content is the battle on the high seas scenario...but that's a whole different rant!)