I have gathered some data from the community concerning the quests after the Icecrown Wrath Gates are opened. This is a preview. I will keep working on this during the next weeks as I experience this myself and gather data that is missing. Be warned that this is a major spoiler. If you are hesitant of purchasing the Wrath of the Lich King expansion, this spoiler might really ignite interest in you in hopping in into the expansion for some very cool questlines involving Thrall, Sylvanas, Bolvar Fordragon, Lady Jaina Proudmoore, and King Varian Wrynn.
Varimathras betrays the Horde. Bolvar Fordragon dies at the Wrath Gate. Varimathras and some Apothecaries coup and dethrone Lady Sylvanas nigh killing her. She and surviving Forsaken refugees reach Orgrimmar. Varimathras reigns Undercity. Lady Jaina seeks council with Thrall in Orgrimmar. Lady Sylvanas and Thrall teleport to Undercity where you must help them retake Undercity. Blizzard uses the new phase-technology which shares the same space as the normal Undercity, just when you get there the NPCs are phased out, and all you see is corpses, and environment changes. You fight Varimathras and his forces in the Undercity Royal Quarter. When you defeat Varimathras, things get even more interesting … King Varian Wrynn and many Knights gallop into the Royal Quarter to face Thrall and Sylvanas. Read the partial transcript. I will add it to the Wrath of the Lich King Story Arcs section (left menu).