I am shitty writer,so i would not write story with Angus send to ASoIaF - but here,short TL:
1.5 years before war of five Kings,he is 20 year old and send to Kastark lands.He sette there
2.War - he kill Mountain when he attack Kastarks during battle,their schieldwall hold,lannister crappy riders run or die.
They attack and shatter centrum.Battle end in draw.
Kastart and Lord Hornwood remain alive.
3.Kastark,as last living son of Lord,is send to their land.Angus is going with him,and Lord Hornwood.
Ramsay attack and die.
4.Kastarks retake Winterfall from ironborns,THeon captured,Rickon Stark saved.
5.Kastarks do not rebel ,like in canon - but,we still have Red Wedding,but this time part of army is not there and survive.
They are going North,install Jon Snow as King,and destroy Boltons there.
Wildlings come as allies.
6.Others never destroy Wall,North independent.