If I get you... In your proposed system generating normal blocks would not be computationally intensive, but generating transactions would be.
Isn't this going the wrong way? This places the weight of the system on merchants with the rest of the market participants simply acting as consumers.
Perhaps, but merchants will always pass along the cost of the system to their customers...
And what's to keep a couple of nodes from making dummy purchases from each other simply to create coin? That sounds more like banking as it exists today IMO.
You can't generate coins by making a dummy purchase. You have to hold a coin to make normal transaction. You would only be able to generate coins by creating a coin-generating block, which would require the same target difficulty as the current system.