Blizz haven't made a habit of releasing 5-man instances in content patches. Magister's Terrace, Dire Maul and Maraudon are the only ones they've ever done outside of game releases. I doubt we'll be seeing any new ones any time soon.

I also like 5-mans. We have a really close group. We've been playing for 6 months to a year now. We have time to do raiding occasionally, but it's much easier as casuals to just get on and jump in a heroic.

I think the problem with new dungeons is the gear. There's enough gear in current heroics to get started in Naxx. They can't really put better gear in a heroic because it's not a raid. The progression is normal->heroic->raid->raid->raid. A lot of people wouldn't run the new heroics because they're gear is already better. Sure they might run it a few times just to see it and the lore but pretty soon they would never touch it.

The asnwer to that is to do what they did with MgT and just make the new dungeons progressively harder. If they can make ulduar 10 man gear on par with naxx 25, they can make some 5 man dungeons that are more difficult than what we have now and have them drop better loot too.