I still say that an awesome thing to do is to go back and retool all raids to have "max level" versions at similar tier difficulties.While I somewhat support the idea of re-using older content... I'd appreciate not losing the original instances as well.
If Blizzard decides to bring more of these old dungeons/raids up-to-date, it would be nice if they'd leave the regular (non-85) version in.
Hence max level versions.
As in, they scale up, but not down. There'd be level 60, 70, 80, and 85 versions of Molten Core, Blackwing Lair, AQs (both) and Naxx (and why not? I'm sure all the original assets for level 60 naxx is still around somewhere, and if not, shame on Blizzard). There'd be level 70, 80, and 85 versions of Kara, Gruul, Magtheridon, SSC, Hyjal, TK, BT, and Sunwell (did I miss any?). And of course level 80 and 85 versions of Malygos, Obsidian Sanctum, Ulduar, ICC, and RS.
Each "tier" would unlock at the new max level in conjunction with the new expansion's tiers (such that they don't have to retool all of the raids all at once).