If one buys Killer Instinct: Season 3 Ultra Edition which has come out this month, he/she will get early access to some fighters in addition to the base game.
According to the listing on the Xbox Store, this edition will cost 39.99 USD and comes with early access to the roster of fighters not included in the base game.
It also includes a VIP Double XP Booster, right retro costumes, accessory sets, bonus character colors, and 18,000 KI Gold.
Since the game is launching on both Xbox One and Windows 10, Xbox One fightstick support will be made available in future Windows 10 update.
Killer Instinct Season 3 will launch on March 29.