There aren't very many interesting mob encounters. The only cool one I've seen is the four skeleton horsemen which is extremely rare but if you have diamond gear they are not much of a challenge to defeat. For endgame players, there should be more challenging encounters like the horsemen or pillager raids but are far more common. One of these encounters could be a patrol of 4-5 skeletons with chain or iron armor but one of them could have enchanted gear or a group of zombies that could easily take you out if you're not careful. In the early game if I come across an armored mob it usually strikes fear in me and makes me have to develop a plan or strategy just toget past them. I never have that same experience in the later game because I can kill mobs in 2-3 hits without hesitation. I don't think this would be a big change to the game and would add a lot of variety to encounters you could come across and I would love to see a small change like this because I am always excited whenever I find a spider or chicken jockey and I remember when my friends and me first saw the skeletons charging us with their horses and it blew our minds!