I'm all about physical rewards:
Soundtrack, T-shirt, plushies, figuers (amiibo or otherwise (a Japanese-style super-articulated figure would be amazing but probably not feasible)), physical game discs, pins, posters, etc.
T-shirt, soundtrack, and plushies would be my top three picks, in that order.
No, not all Xbox owners are dudebros who love grey-and-brown PvP FPSes. Please feel free to quit stereotyping now.