First, they need Ritual effects
Still, some version of this effect could be a cool enabler, and I would love to see this as a combo piece in a Laboratory Maniac deck
Keep in mind, though, that whatever you pay for X has to include the original conditions of that creatures mana value
With some heavy defense Illusion cards, players can stack up a good wall of defenders that can be used over and over again
Callidus Assassin Interestingly, this is another Assassin from a Universes Beyond set
As far as R&D is concerned, it is the theme discard cards that cause the most unhappiness as discard decks are one of the scary three (land destruction, permission and discard) that tend to keep people from playing the game and thus tend to annoy players at a much higher level than other decks