More speculation, but it seems logical that Fame could be used within the system itself. In other words, use Fame to purchase more Mission Creation items like larger maps, more complex objectives, signature NPCs, etc...and of course badges.
Positron likened it to the Character creator in his first announcement of the feature, so I think of the mission features like the Vanguard items. All the options are built into the system, but you have to basically unlock them by getting your Fame to new levels.
I may be wrong on this, but I really don't think the Mission Creator will give normal game rewards. If they do, each mission will need some sort of "cool down" period to lessen any exploitation of the system. I'm still not sure they would even do that, though.