1 - They should replicate the city of Las Vegas and surrounding suburbs, building for building, street for street, and make an open-world game like Ghost Recon Wildlands.
2 - They should make 22 maps, or more than 22 maps for PvE terrorist hunt, 1 player with 3 AI teammates (or up to 3 friends). AI teammates are very important for terrorist hunt. Rainbow Six Siege is too difficult for PvE terrorist hunt, because you have to go in alone, and the supply crates only give back ammunition and do not give back health.
3 - They should include mapping tools, so that the gaming community can make custom maps for PvE terrorist hunt, like how there were hundreds of PvE maps for Raven Shield, all made by gamers.
In addition, they should make it easy for people to add custom maps into the game, by providing a modding interface similar to what ARMA 3 has, or Total War Rome II.
4 - They should release the game on Steam, (not Ubisoft alone), so that people can use Steam Workshop, and subscribe to custom maps, which results in the maps being added to the player's computer.