Personally I would love to see some unlocks referencing the older games such as a Silent Assassin suit (from H2SA) a Contrats suit (from Hitman Contracts) as well as some cut challenges from Hitman (2016) and Hitman 2. I know that the community and I have been wanting these cut unlocks/challenges/escalations. I would also recommend some unlocks and challenges I made especially from this post: Hitman WOA - Challenge Ideas - #186 by BRO3886
If anything I know having the Hitman Absolution promo figure would be epic. Could even be a twitch drop 
