You know what sounds fun? Getting rid of the 2 mana restriction on Isochron Scepter, and making it a creature. In Magic 2014, they did that, and gave us Elite Arcanist. Unlike his Scepter predecessor, Arcanist has no limit on the mana cost of the Instant he flings about willy-nilly, so you can happily put expensive and fun things on him to use every turn. But how does he work with X spells, like Sphinx’s Revelation? Do you get to pay mana into X? Do you get to decide what X is and get it for free? The answer to both is, sadly, no. Outside of the stack (which includes exile, where Arcanist keeps his spells), the value of X in a mana cost is always 0. So a Sphinx’s Revelation underneath your Arcanist always has a CMC of 0, and it’ll cost 3 mana to use Arcanist to cast a copy. The problem is, since you’re casting the copy for free, you aren’t ALLOWED to put anything into X, it must be 0. So Arcanist doesn’t work very well with X spells, since they’re not likely to do much! Do note that you’re still allowed to pay ADDITIONAL costs when you cast your free spells from Arcanist, just not alternate ones.