I'm reading the OP's post a bit differently.
If the normal function of a player is to produce an army, build buildings, and research technologies, then I think what the OP wants to do is once a week give a player a major, non-direct boost. For example, if the player has a building that produces research points, then balance the game such that every week over a 12 week period, the player makes a 'major' technology discovery that does something like produce 50% extra grain from farms.
I actually like this idea very much as it provides a fixed ratio reward system for the users. And users like rewards. I think I will use this...