MaRo goes over all of the decision making processes for UB sets, particularly how they choose which other IPs to go with:

• Overlap with Magic players or potential Magic players
• Elements that translate well to Magic cards
• A partner that wants to work with us

He also spends a good bit time discussing how they “[make] sure you capture what people love about the property.” For WHO this became a variation of Partner for commanders partnering with a “companion.” Kind of annoying they went with that term given companions are already a thing from Ikoria!