I did some more researching the A-10 replacement. Apparently even they haven't decided exactly which way to go. The F-35 can't do the A-10's role (personally I think the only unique aspect about the F-35 is it's ability to VTOL. Though helicopters over the past few wars have shown how vulnerable that makes them). The could design an new plane from scratch, or they could upgrade the Warthog.
IMHO I think any advancements that can be "bolted" onto the plane would be worth looking at. If they try and redesign the A-10 as they go it will probably do more harm than good. New missiles, smart bombs, better ammo are good starts. New computers and plan/pilot interfaces too. But modifying the planes structure isn't smart. It's hung around precisely because of the advantages of the airframe. Don't mess with a good thing. I would say maybe save weight by replacing panels with carbon fiber but after much thought I don't know how much that would save and how much it would gain the aircraft.
We have seen the prevailing mentality of upgrades with other aircraft.
The F-15 was introduced in 1976. The original design was a hot rod of an aircraft, setting almost a dozen time to climb records with the "Streak Eagle" project. On one flight the plane reached almost 99,000 feet in 3 minutes and 28 seconds. I calculated the speed of the plane at around 1,000 feet per second in a near vertical climb. Now - 40 years later - there's the F-15SE. An F-15 with newer engines and external conformal fuel tanks (known way back as the "FastPak" system) and all sorts of crap hung on the plan. Supposedly more stealthy but no longer the dominant dog fighting jet it was.
The F-16 started out as a relatively cheap superiority fighter that would supplement the more expensive F-15. An unorthodox (at the time) design that proved to be highly maneuverable. The USAF Thunderbirds don't use the aircraft just because it looks pretty! But it do suffers form a lack of a worthy replacement and so has been "upgraded" with more weight and equipment so that it can't even turn inside a Mig-29.
The F-18 is the most obvious. Starting out as the YF-17 lightweight competitor against the YF-16, it was a really unorthodox design, though it didn't quite meet the demanded specs (at the time). But the Navy was looking for a lightweight fighter to supplement its F-14 Tomcats, and the YF-17 design was much more adaptable to the Navy's needs. It was so redesigned it was given a new designation - the F/A-18. Much heavier than the YF-17, it proved extremely agile and adaptable. The Blue Angels fly it. I've seen them, and have since gained alot of respect for the F-18. Well, over the years more demands have been demanded of it so the F-18A has been enlarged and upgraded to the F-18C. Then the Tomcats were retired, and the Navy was left without a frontline heavy hitter. So they "upgraded" to F-18 again to the E Super Hornet. There's no way it can equal what the F-14 did, but, what can they do? Make it larger again, which needed bigger engines, which required extensive redesign of the intakes and rear end, and thus heavier landing gear. And it loses most of it's original advantages!
With the A-10, it doesn't need to be faster, or carry more bombs, or more durable. Just build more of them!