[Fish]

• Something beyond the stupidly simplistic "We're going to kill everyone and make a better world." They never do anything to further his agenda beyond 80's cartoon villain nonsense. They never flesh out any of his personality or thoughts. He feels completely disconnected from the world he seems so intent to destroy.
• Maybe show an actual compelling reason for his followers/Horsemen to actually fight for him. Because through the entire movie, the only ones who actually remained loyal were the initial Horsemen that died protecting him during the initial uprise against him. The current generation of Horsemen were each like "Meh, I'm done now." Which made his ties to his Horsemen confusing and weak.
• Maybe show something a little smarter than his Short Circuit-esque "TV watching to gain the entirety of human history" BS, or as Apocalypse described it "Leeaaaaarnnnnnnnnning" Which was as dumb as the old Lightning Toad quote. And there was a disconnect between him wanted to destroy humanity, before he actually knew anything about humanity over the last few thousand years.
• His outfit/makeup made him look more like an awkward old man with purple skin, than anything close to the most powerful mutant ever. Nothing about him evoked much fear or intimidation.