This is the main thing I think people need to keep in mind when thinking about guests. According to the documentary on The History of KI that came with the Definitive Edition, Adam Isgreen had been trying to get The Arbiter into Killer Instinct since Season 1. If it took them 3 years to get access a character that their parent company already owned, imagine the amount of time and cash they’d have to sink in to get a character from a franchise they don’t even own (that frankly doesn’t have much current cultural relevance outside of memes anyway).
The cross-overs that we got happened at least partially due to good timing and cooperation with other Microsoft owned companies. Rash was announced with Rare Replay, Arbiter during the Halo Championships, and RAAM right after the Gears of War promo at E3. From a business perspective, Microsoft would need monetary incentive to give the green light. I don’t think the Guy Fawkes mask being a popular Halloween costume is gonna provide do that.
From a gameplay perspective, IG would need a place for a character in the roster, and quite frankly V’s role has already been fulfilled. Leader of a rebellion against an oppressive force? Orchid’s got that covered. A masked bringer of chaos? Shadow Jago has been a real boy for a long time now. Sword fighting? We’ve got 3 characters covering that. Dressing up all dapper-like? Already got plenty of fancy accessory sets.