Ok, what does your low RCS (that'd be stealth) A-10 look like?
Let's put it at medium altitude so that we don't have to try to armor it against manpads.
Then we can probably make it single engine, so we can get a bit cheaper mx.
Let's skip the fuck off huge gun, because it's raison d'etre was killing tanks, but even when it was designed, it was only good against the rear or side of a tank, not the front, and that was 50 years ago. If we use a smaller gun, we can save the weight for more useful things, like guided bombs.
It's going to need much better sensors, since we want it at medium altitude, so let's make sure to integrate them.
If we make it supersonic capable, we can increase the radius it can work over when on station.
I'm not a professional airplane designer so bear with me. This is drawn from my reading of the textbook "Aircraft Design: A Conceptual Approach" by Daniel Raymer.
The challenge is you want something that can linger and pick at targets on the ground with ease. This necessitates a low stall speed, requiring the use of a straight wing. The aerodynamic characteristics of anything resembling supercruise demand a swept wing, which would destroy the "lingering" aspect. So you can't get a good CAS with an airframe that works well for a fighter role.
The gun on the A-10 is useful for strafing runs against infantry, not just concentrated fires against MBTs. To do that, you need a substantial round + LOTS of ammo. The F-35 just doesn't have enough internal storage capacity to service this need. So you've got to keep the gun. It also has barely any external mounting capability for bombs, rockets, and missiles.
So, I'd say it would be something that looks a whole lot like an A-10 but with CAD assisted fuselage shape changes to minimize RCS + new materials + new avionics.
I have no idea how to mitigate MANPADs as I'm unaware of what they use for targeting solutions. The distinct twin-tail on the A-10 was picked to provide a shroud for heat emissions from the engines (c.f. Raymer) to mitigate those types of attacks but I imagine that near peers have increased their capabilities.
If you're moving at medium altitude, you're moving relatively slower (in angular change) for the same airspeed, so you don't need to fly as slow, and you don't need a low stall speed. You just need the ability to carry enough fuel to match your desired loiter time. Most CAS is performed by fighter aircraft at this time, indicating that your supposition that fighters can't do good CAS is false. Also, "supercruise" is a specific term indicating the ability to stay at supersonic speeds for a prolonged time. Most supersonic aircraft can't supercruise, as they don't have the fuel to do so.
The gun may be useful for strafing runs, but it's explicitly designed as an anti-armor weapon, and it fails at it for modern peer tanks. The JSF cannon is still a 25mm cannon, but it's much more optimized for explosive effect, which is more useful than the 30mm anti-armor cannon that the A-10 carries. There may be call for having more ammo (the A-10 carries ~6x more ammo than the F-35), but practically speaking, the A-10 has 9-18 bursts (1-2 second bursts), while the F-35 has 9-10 bursts (~20 round burst). But again, you're supposed to be doing CAS at medium altitude, which means your gun is more of an after thought.
Beyond that, the F-35 has more external mounting capability than an A-10. The F-35(A/C) has a listed external capacity of 15klbs on 6 external stations (5klbs on the inner stations, 2.5klbs on the middle stations, 300lbs on the outer), plus the 2 internal bomb stations (2.5klbs each), 18klbs total. The A-10 has 11 stations, but not all of them can be used at the same time. The outer stations are spec'd for the same as the F-35 (300lbs for AIM-9). But you can use either the side by side or the center station under the fuselage, and the wing stations aren't spec'd as high, for a total load of 16klbs, or 2000 lbs less than the F-35.
MANPADs are mostly IR, but newer ones are imaging IR sensors, and can attack from all aspects. The A-10's shrouding helps against older missiles, but not so much against new ones.
Let's put it at medium altitude so that we don't have to try to armor it against manpads.
Then we can probably make it single engine, so we can get a bit cheaper mx.
Let's skip the fuck off huge gun, because it's raison d'etre was killing tanks, but even when it was designed, it was only good against the rear or side of a tank, not the front, and that was 50 years ago. If we use a smaller gun, we can save the weight for more useful things, like guided bombs.
It's going to need much better sensors, since we want it at medium altitude, so let's make sure to integrate them.
If we make it supersonic capable, we can increase the radius it can work over when on station.
I think I'm back at a F-35 now.