I think the show may be a summarized/cliff notes version of the Battle of Ice. Some desert Stannis' already weakening forces and he loses to Ramsay. Stannis dies a ignoble death.
Then, Sansa then shows up with an army from the Vale. Manderly then turns cloak to Sans's side and it allows a slaughter of the Boltons, as Manderly's men allow the Vale knights easy access to Winterfell, Sansa marches in as the new Stark in Winterfell.
Patrick Malahide, the actor that plays Theon’s father is 70 years old and does not look like a young man. Ian McShane is 72, but looks younger than his age. McShane could easily play Malahide’s younger brother Euron.
McShane also has the intensity and gravitas to play a harsh & demanding military father, like Randy Tarly.
I highly doubt he would play another Greyjoy and somebody not named as a member of the upcoming season 6 cast.
I'm going to pick up the hardcover and the audiobook the day of release - I'm too old for a midnight release party, unless it's on a Friday night into Saturday, or Saturday night into Sunday. Not sure how long it will take me to get through the book, but I'm guessing a few weeks. I doubt I'll be able to avoid at least some spoilers.
A few episodes back, Stannis said he had 6,000 men when he was in the tent with Davos before the blizzard. Then, the Stormcrows left (Damn you Daario!!), and that was a loss of 500 mounted men, per Stannis' quote in the episode. That left him 5,500. After the Shireen burning, half of them left, which leaves him with 2,750.
It seemed like prior to the burning, most of the losses were to the horses, but we should still figure there were also a few desertions and the sentries Stannis had hanged. So, maybe 2,600 to 2,700 in total before the battle.
Good move by Roose to attack, as each Bolton loss is one less mouth to feed through winter.