Everyone has looked at a villain now and then and thinks to themselves "if they would just do this..." so lets hear your ideas on how to improve an underdeveloped villain or one that has had way to many bad storylines that have kind of ruined them.
For me I am going with Apocalypse. They have really wrecked that character over the years with bad retcons and just dumb ideas. From making him just a parasite that jumps from body to body to making his whole mission just things the Celestials programmed him to be. I would strip it all away and bring him back as he was before the Celestials altered him. The ancient warrior who lives by survival of the strong and the culling of the weak who has no mercy. An En Sabah Nur following his own path for the first time in thousands of years and not just doing what the Celestials led him to do. Sort of like he was in the Rise of Apocalypse origin mini series.