In the A Song of Ice and Fire series, why doesn't Ramsay Bolton try to take the Iron Throne for himself? He could use his treachery to his advantage and mostly certainly have a chance of winning by playing by his own rules. Especially after Tywin’s death, the Lannisters wouldn't stand a chance. It seems like a real failed opportunity.
One step at a time now...
Currently the Boltons, specifically Roose, is looking to make himself King in the North (or so is suspected).
"Truth be told," she said, "Lord Bolton aspires to more than mere lordship. Why not King of the North? Tywin Lannister is dead, the Kingslayer is maimed, the Imp is fled. The Lannisters are a spent force, and you were kind enough to rid him of the Starks. Old Walder Frey will not object to his fat little Walda becoming a queen. White Harbor might prove troublesome should Lord Wyman survive this coming battle … but I am quite sure that he will not. No more than Stannis. Roose will remove both of them, as he removed the Young Wolf. Who else is there?"
A Dance with Dragons - The Prince of Winterfell
The Game of Thrones is long one to play... and at this point while Boltons may have enough strength to take the North they would not be able to march on Kings Landing.