Sion was one of the champions available for the Noxian pool during the Ionia vs. Noxus match, and was one of the selected champions.
Feel No Pain was possibly taken from the Warhammer 40,000 table top game, where it is a special rule that gives units a chance to ignore wounds.
Sion is one of four champions that can infinitely increase their stats by using their abilities, in Sion's case it's his health. The other three are Nasus, Thresh and Veigar.
Sion's character model now shows him wielding a double-bladed axe, as he does in his Warmonger and Barbarian skins, rather than the classic single-bladed axe that still currently appear in both of his classic skin artworks.
Lumberjack Sion skin could be a reference to Paul Bunyan, with Unchained Alistar as the reference to Babe, the Blue Ox.
Lumberjack Sion may also be a reference to Mark Kaminsky - character from movie Raw Deall, played by Arnold Schwarzenegger.