I think missions would be nice, but I like the way of having open creativity- free as soon as you start the game. So, unlocking markings and stuff I don't agree with. but I think each player could have a journal, and there could be a checklist of the missions you have done. In multiplayer you can choose to show it, or not to show it. c: I think that would be pretty nice. ^-^
Also! Maybe there could even be two mission modes, life mode, and instant mission mode. Here are how the two modes go...
1. Life Mode- This is a mode, where you can start out as rather a cub, or pup, or whatever species! You will get a random amount of litter mates, and you have to go on certain missions as a pup, and after you complete a certain amount of missions you grow. And onwards.
2. Instant Mission Mode- You get to pick from a list of missions you have already completed and you can re-do them as easy, medium, or hard. Then you can run that mission at whatever age you want.