The last time they introduced a new Worlds Championship, the inagural champion was Finn Balor immediately after his callup. He got injured and they gave it to Kevin Owens (an unproven act in the Main Eventer at the time), who held it for 6 months. Neither reign did much to elevate the new championship.
Due to this I would be very hesitant to put it on an unestablished Main Event level performer. This limits us to minimal options.
If I were WWE I'd put it on Seth Rollins, a bonafide main event level guy that's incredibly over, if for no other reason than to build up the credibility of the championship. But Bob Lashley would also be a good choice, with Drew McIntyre also a good candidate.