I have give this a little thought. The A-10 can be replaced.
With the proliferation of precision guided munitions, I think the A-10 could be replaced by a aircraft with a bomblet dispenser. Its primary weapon would be 25-50 lb. guided bomblets, and it would carry dozens, if not hundreds of them. The bomblets would need some standoff (glide) capability, and would be available in multiple types, including armor piercing, HE, smoke, gas, etc. It should have a crew of two (pilot and weapons officer), able to interface with ground pounders, and able to rapidly engage large numbers of targets simultaneously.