There are a few ideas I've been thinking could be an improvement:
1. Ice Light. With modern LED lights and projection lenses, you could have a dual head light that would both light up leading edge as usual, but also light the ground/sides under the wing. This would create great situational awareness during night operations. One of the problems with most aircraft is that there is no side lights and at dimly lit airports with no edge lighting on taxiways, it can get confusing quickly. I almost drove my 520 into a ditch on one at night.
2. The aft nacelle "tip" behind the wheel well on turbine Commanders is just sealed off dead space. Surely a little lid could be developed at top so that that space could be used to keep tools or smaller items in?
3. An aft baggage compartment aux tank. If you wedge it right behind the pressure bulkhead, below collector tank, it should have minimal effect on CG. You could have a transfer pump that pumps it into the collector.
4. What about a little Cub-like sightglass fuel meter on the collector tank? Since Commanders have notoriously bad metering, maybe that would be an extra safety measure. You could rig it so that a little pinhole camera films it and then have a small screen in panel.
Just a few ideas.