Many roles are only viable when they have another role in play. The Cthulhu is the most obvious example, but nearly every role benefits from synergy.

After selecting "Closed Roles" in Game Options, you should be able to click on the icons of two chosen roles to establish their coexistent tendency -- choosing a percentage between +0 and +100.

Having this set to "+100%" would guarantee the first role would always be accompanied by the second (but the opposite wouldn't have to be true).

Tendency could balance Sandbox and make some roles more useful. Furthermore, this RNG manipulation wouldn't have to be too noticeable, per the host's decision.