2) Interactions - When you add in the "Powers & Feats" module it is going to have to provide mechanical integration with other modules, like defining the need for rests. These will be deeper and fiddlier as well, think about all the issues between classic and Essentials 4e classes and MC feats and power swaps of various kinds. These things will just mushroom with the number of modules, and because each module is designed to be isolated from each other module it is difficult for them to resolve these things around specific resources or mechanics that may not even be in use. How will Rituals leverage APs when APs are an optional part of the "Advanced Combat & Tactics" module?
See, this is why i'm talking about folding and unfolding different kinds of 'Points' on that other thread, and the other issue relating to it.
The key would be to ensure that the various systems interact through a central point exchange system, and then the DM would decide which systems were used, and how points interacted and did not interact.
By doing this, and by also, for isntance, ensuring subsystems used similar mechanics and concepts, it would be viable to balance various systems for things like say, combat, grand ritual magic, and kingdom building.
So your kingdom might net you some resource points, which you could spend on up to half the cost of the grand ritual you're casting. The other half of the cost might come from drawing on your party's life force in the form of wound points(a healing surge like resource), and that will make fights harder for some time. OTOH, in another game you might just cast the ritual with a more general 'hero point' value, which would also play various other roles, while a third game might require a more detailed gathering of magical resources- but they'd still be defined as points, and plug into the system in the same way.
OTOH, if those systems interact in a messy, detail-oriented way, then the interactions just become massive and unmanageable. I can't imagine the meals additive approach being as simple to navigate as one where the various options and subsystems are more deliberatly 'baked in' and designed to interact only in clear, simple, and managable ways.