Given the magnitude of potential savings, I would assume large airlines have the funds to hire competent Operations Research folks to balance the undoubtedly large number of variables that go into schedule planning. It would absolutely floor me if any of us here would be able to think of a solution in a few minutes of casual day-dreaming that staff working full-time on optimization hadn't already explored. I would also expect that even a single percentage increase in utilization would be considered a major success.

As just one observation, you implied that a given airfame operates back-and-forth between a hub an an outstation. I don't have a handle on what fraction do that, but I rarely see that pattern. Instead, I see flights go from one hub to an outstation and from there to a different hub, so suspect that your hypothesized two hours of idle time at the outstation is where efficiency gains have already been found.