Playing into the poker and high noon shootout tropes of the wild west.

I'm not sure what this needs to cost to be balanced. I'm also not sure if it's fair that everyone else discards the cards, as that's a double-hit. But if you don't do that, there's no tension as everyone just plays their biggest card and hopes for the best.

Right now you just use this to cheat out Emrakul, and still get the extra turn. The only things that'll stop you are counterspells, Draco, or a tie with Autochthon Wurm (and it's unlikely that the opponent will use the latter two without a reason to build around them). Since the most mana-intensive effects are usually also the swingiest in terms of Win Probability (at least once you clear the hurdle of casting and resolving them), this effect simply doesn't balance well.

Good point, any "play for free" is going to have similar problems. Any better suggestions for something that would work in a similar way? Something like choosing a card from the top N of the library?

Whoever reveals the cheapest nonland card. That way, each player wants to cast the biggest thing possible, but they have to guess how big the other players are going to try and be smaller than that.

ROFL. Yeah, that's pretty good.

OK, since Showdown is a mechanic, we don't need a card for it, but we do need some splashy rares.

This is a bit similar to Go for Your Guns!, so I may change it, but the basic idea is to have a high-stakes showdown at a larger mana cost and rarity.

Not sure about size of effect and cost.