For me, I really really want to see the Alliance begin to rebuild the ruined kingdoms of the 7, then reunited them all. Basically would involve the whole continent of Lordearon. Would be the perfect time to introduce Kul Tiras (given all the talk of the 'next' exp), could bring home Danath to make an offensive to completely retake and rebuild Stromgarde and start to cleanse Gilneas so it's inhabitable. The only kingdom I'm not sure what to do about would be Alterac given its checkered past.
This would also be the perfect time to advance the stories of the Forsaken and Quel'dorei as I'm sure they'd take issue with a strengthening of Alliance forces in the north.