[[SPOILER]] I think I know why I think Kaiki looks younger now. I remember him looking pretty tired all the time and now I don't see any signs of being tired on his face so that might be why, or maybe I just remember his face wrong.
It kind of is spoiler-ish but I felt like it was obvious he was going to be back. He's played quite the role in the monogatari series I don't think he would just disappear. He's quite deeply intertwined.