I'd stick with single turbine-single prop. Before I'd use a gearbox for contra-props, I'd go for twin turbines and coaxial props. That would add complexity, weight, and cost ... so the first choice would be a single turbine.
I'd put either two Vulcans (or similar) gattling guns fixed or four 20 mmm cannons fixed, and either one or two chain guns slaved to the pilot's helmet. If one, the mount would be centerline in front of whatever centerline pylon is mounted ... could even be in the front OF the centerline pylon. If two, the mount would be inside the wing with the chain guns protruding out underneath the wing in relatively streamlined installations. I'd allow for as much as 90° of travel off-axis. Alternately, they COULD be mounted at the wing leading edge, allowing for training the chain guns UPWARD as well as downward. In certain circumstances, it could work to a huge advantage. and the firing arcs could be electronicallty connected to the propeller arc to prevent shooting off your own prop or even the pilot.
With 5,000 to 6,500 HP, the weight-hauling ability should be pretty decent and I'd expect (really) about 400 mph or better in clean condition and 350+ mph loaded, with about a 6,000 to 7.500 feet per minute climb rate. If the two-engine configuration were selected, one engine could be stopped and the prop feathered to extend loiter.
One feature I'd add would be to put in a lot of ammunition, especially for the chain gun(s), so that the extended loiter would not be wasted by quickly running out of ammunition. That would cut down on ordnance, but would allow for extended ground support in a guerilla warfare environment. A possible alternative would be to come up with a wing-mounted (on a TER or MER) ammunition tray that could be mounted next to the gun to extend the ammunition only when necesasary for the mission. Like the Skyraider, I'd use conventional landing gear, not tricycle gear, and would fit it out for Naval operations if requuired. That means folding wings and arrester hook and catapult-launch capability.
The overall size would be about the same as a Skyraider or slightly larger, depending on weight when the design was developed. Smaller is better as long as it remains maneuverable, tough, and rugged.
I'd mandate a symmetrical airfoil on both the wing and the tail so that wing panels and tail stabilators could be interchanged for battle damage repair. I'd skin it with .050" or thicker 2024-Aluminum for ruggedness and stress it for 8g at mnaximum load out (which would mean 12+g at light weights ... though "light" might be relative).
Speed brakes very similar to the Skyraider would be fitted, too. I'd also want a propeller brake, and the plane could have the engine running with the propeller stopped if the turbine were selected to be an indirectly-coupled unit. That might or might not prove useful in normal circumstances, be WOULD in a quick-turn rearm in a hot combat zone.
Just some thoughts about what would be effective today ina limirted war. In unlimited war it could also be effective or could be irrelvant if the Earth is destroyed by nuclear arms.
The concept can be argued as good or bad, but the product has absolutely shown itelf to be useful in real war. The Skyraider was only retired when the R=3350 engine became difficult to service and keep running with the available spares. If the new plane were modular, worn or damaged sections could simply be replaced at any depot. If we mandated an engine mount taht was, say 15 - 20% bigger than needed, then the engine compartment would allow for growth engines to be fitted as well. That adds a bit of otherwise unnecesary wetted area, but we're only talking 400 - 500 mph anyway, so the ectra area isn't very critical.
All just an idea that will probably never see the light of day other than in here ...