I saw a comment anywhere I can't remember where, I think it was on this same forum or another, that said the final villain might be Ironside, I decided to post this theory here and also supplement with my own points, but basically the theory started by drawing attention to the fact that Vargnese says that Ironside betrayed Arthur, I had forgotten that, and probably several people too, if Ironside betrayed Arthur then it is likely that Arthur doesn't know and Vargnese somehow knows, maybe he knows why he knows Meliodas? Arthur trusts Ironside more than anyone and I doubt he would if Ironside had betrayed him before, what if Ironside has been behind it all from the start? What if he was responsible for Arthur's change? Maybe he's the ultimate villain, maybe he wants the chaos powers, and after taking the chaos powers, maybe just a part of it, but more than arthur, maybe at least 75%, so Arthur would join 4KOTA and the 5 would defeat Ironside, I know there are some holes in this theory since Ironside really seems loyal to Arthur, but anyway, add and correct some point if you want.