Why not offer new/better prizes for playing and completing levels in the game. Offer mod's once you finish all 5 steps in a level. Maybe this could be a route for people to get mod's they didn't previously acquire or perhaps throw in a "surprise" factor whereas you would get a secret building or secret weapon. Perfect example is upgrade +1, newer players are pretty much hosed on that.....if you offered it as a mod for finishing a few levels in the game that would reward players for progress/strategy of gameplay.