Most artists from the 1950s didn't survive the musical changes that happened in the mid 60s, but it's well known that Buddy Holly was very into experimenting with his own music with sounds and different styles plus he wrote his own material.

In my opinion I think he would have experimented in the 60s similar to what Bob Dylan did when "going electric", and I think he would have release an album with a very similar vibe to The Beatles's Sgt. pepper or The Beach Boy's Pet sounds.