FR the Targaryens needed less Valyrian magic and more institution building. A permanent bureaucracy. A standing national army. A legislative council. Make more lands into royal fiefs. Do something to turn this confederation of warlord kin-groups into some kind of real cohesive nation under a real state.

There's a scene in either Feast or Dance where Cersei is trying to borrow money from the Iron Bank and she thinks Westeros needs its own bank, and it's like the best idea anyone in these books has ever had. If she could have gotten that started it would have been a huge plus for her in the history books.