In reality, "anywhere on the system" should have an asterisk. That's because the maximum number of miles you will get, regardless of airline and planned destination, is 60K per ticket.

So, as freeupgrade suggests, give Marriott a destination that will get you those 60K miles per ticket. Once the miles are in your airline FF account, you make reservations to the destination of your choice based on the rules of that particular program (i.e., whether stopovers, open jaw and other special requests are allowed).