This is the approach I'm also looking at. GTD systems are good at storing and keeping track of times and due dates. But the planning/execution step is missing b

This is the approach I'm also looking at. GTD systems are good at storing and keeping track of times and due dates. But the planning/execution step is missing besides basic "work on the highest priority task for each project. The next step could be a scheduler that takes the GTD state as input along with your calendar and builds an execution plan.