what about a new type of artifact. Books or scrolls or something, that activate a few turns after they're put into play. They could be cards that have a large effect on the game, say for example. "Rename. Costs 5 During the upkeep after the following you may at any time after that have all creatures that target player controls become the creature type of your choice. Then sacrifice Rename." So basically they would be an instant you could play one time, then activate another, after a certain perioud of time.
It is not inconceivable. My question would be "why?" If you're going to make a new subtype, there should be a good reason, like equipment. Making the subtype cut needless text off cards that used it. How does your idea do that?
