*UPDATE*
After reading through the comments, and from other threads, I understand that Naxx SHOULD be the last raid. The content I'm suggesting is to compliment existing content.
-Grim Batol would drop T2-2.5 quality items, with most gear falling towards T2.0 quality. The last few bosses would drop some higher ilvl pieces.
-Nothing from Hyjal would obsolete existing content, but compliment it.
-The smaller outdoor 5-10 player dungeon in Azshara would drop T1.5-2 quality items.
-No one seems opposed to the existing, but not implemented Azshara Crater BG.
This is something I've always thought of. Ever since the Cataclysm, I've felt that the storytelling was slowly but surely taking a nosedive. With all of the pre-Cataclysm unused areas/portals, what sort of content would we have gotten if The Burning Crusade had taken a few more years?
Grim Batol Raid
Since the dungeon was added in the Cataclysm, it wouldn't take as much work as creating a whole new area. Much of the actual architecture can be reused or straight copied, then re-populate the dungeon to suit a higher tier raid, with new bosses.
Hyjal opened as a quest hub
Hyjal as a quest hub could revolve around the beginnings of restoring it to what we see it as today on retail. Running around the demonwastes, trying to fertilize the area, and cleanse it of any remaining demonic presence. Hyjal is already there, it just needs to be populated, and the quests made.
The final available quest in Hyjal would be the beginning of an attunement quest chain for another new raid, The Emerald Dream. They could finish off what they had of the vanilla Emerald Dream, and deliver it to us as a mini-raid. Lore wise, perhaps we're investigating why the 6 portals to the Emerald Dream on Azeroth don't work. 1-3 bosses.
Azshara (the zone)
Finish it, obviously! They could also finish off that Azshara battleground that's been on the shelf forever. Azshara would be a decent place for a max-level outside 5-player dungeon, or maybe even a new 10-layer dungeon/raid thing. The purpose of this dungeon would be similar to that of Diremaul, which was gear catch-up. Make it more difficult, so players would need BWL level gear to clear it.
And finally...
CLASS CHANGES
NONE.
I think they're fine.
Seriously.
A lot of the issue with specs in vanilla was itemization. This can easily be fixed with new content. A few good items designed specifically for feral tanking or dps, moonkins, ret paladins, prot paladins, and shaman dps. Either through stats, item specific effects, or even set bonuses, the specs we currenly have in vanilla are already fun - and plenty viable. I'm not an expert in what stats precisely would make each spec more viable, other than some ideas.
Ele shaman - They could make definite use of either additional mp5 on gear, or a set bonus that grants mana back on spell crits.
Enh shaman - Without dual-wield, Enh shamans are still a little limited. A set bonus that removes the proc, but guarentees a Windfury every 4 hits could be cool.
Moonkin - Between the high cost of wrath, the slow cast of Starfire, and the lack of *umph* from both, Moonkins need more than a little help. However, if more feral tanking gear is added in, Moonkin could be seen less as a "caster dps" and more of a true "caster tank". The Panzerkin.
Feral Tank - More leather itemized towards tanking, and a nice weapon. Bears are already barely just not viable.
Feral Cat - A proper weapon that isn't from a level 30 dungeon boss, and set bonus similar to the Wolfshead Helm effect.
Ret paladin - All they really need is a good 2 hander with STR and Spellpower.
Prot Paladin - A few STR/Spellpower items including a mainhand would certainly help.
Obviously, doing something like this would require new servers for people to play on, or at the very least, changing a few of them over, but offering free transfers away in order to preserve the integrity of your vanilla experience.
Does anybody else have any brain droppings they'd like to share?
And in case anybody cares, I played a warlock.