Automatic
This is somewhat inspired by that new Hearthstone card that automatically suggests cards. How this card works, is that it looks at your hand and board, and replaces the card it is least inclined to replace if it were an AI opponent. For example, this could replace your 4 cost play if you played this on 3 mana. Or this could replace your board clears if your opponent has a big board. Since this is a start of turn effect, your opponent gets to suffer from it first. In some cases, you can turn the downside into an upside since you get to replace an additional time, even if its a terrible replace. I envision this being used to replace your opponent’s completed Mythron Wanderer. Other Mythron are releatively safe from it, though, since incomplete Mythrons cannot be replaced, and mythrons are usually played on/1 turn after completion. It does feel kinda overpowered, so I might nerf it to 4 or even 5 mana