A lot of elements of TBC were originally planned for classic wow, as has been stated by other posters here.
I would be so down for Kara in vanilla, but not as a catchup raid, but rather as a post naxx raid.
I also don't think they should go to TBC after vanilla. I know a lot of people want that, but I would prefer vanilla to stay vanilla with added content later on. I love the TBC talent tree design and would love to see some form of implementation of that, or even new spells and abilities as well, like treeform form druid for instance. But level cap should not be increased, so how they would do this is up to them. If they want to make some paragon system like they had in diablo 3 sure, although I don't think that would necessarily be the right call.
More classic content in line with the classic values is what should happen.
Horde | Druid | Incendius | Cowligula