how about either as follows, but not more than one choice if it's once a week:
+1 to all personal stats
300 max vp and ve
1 or 2 md shop credits
Once a month I think would be more along the lines of what killer shows, also add possibly:
random creature... with this feature you can choose to take a chance at getting a new and unique creature... what to do... make 2 unique creatures for it... then add all current creatures in the game to it and like the lottery pull out a random creature i.d. and that'll be what you get. odds of getting unique creature should be 1/100 or so. This way the creature will be that much more special to get.
+3 to all stats(modifying killer's first option for once a month)
I think you should only get to choose one of the options.
Also, stuff can become greater as more people participate.