A replacement for the heat shield system, I don't know of. However there was a planned, and mostly completed, replacement for the entire Space Shuttle, called the X-33/Venture Star.
I've done a fair amount of reading on the after-incident reports of both Challenger and Columbia. And as far as Columbia goes, in my opinion, I believe that the hardware takes far too much blame where the personnel (human negligence, not human error) should. Long story short, the risks to the thermal shielding were well known for years and many missions before Columbia. I believe that people were more concerned about keeping their jobs, rather than taking precautions that would have slowed the program even more, or shut it down potentially. And then there are the contractor production and payload revenue influences.
Regardless of what I say, the professional review board for the Columbia destruction put most of the blame on NASA's risk management and flawed operational "order of battle" (my paraphrase, not theirs).
The damage was known long before re-entry. No one knows if the crew could have been saved, but it is known that we didn't try. We had the ISS in orbit as a potential safe haven. We had the Russians to bring them back from the ISS if we could not get another shuttle ready in time. As galling as it may have been to ask for help, it's preferable to crew death. I think if we had shown the resolve that we did with the Apollo 13 accident, that we could have saved lives. Some egos and maybe some careers would have been bruised (which ended up getting bruised anyway), but that shuttle crew may have survived. Not even an attempt was made.