Thoughts on Creating Specific New Cards

Mulldrifter:

-Start by creating the 2/2 flying creature for 4U.
-Add the cost reduction code (like affinity) to reduce its cost by 2.
-Add a comes into play ability. Part 1: draw 2 cards.
-Comes into play part 2: If 2 mana is available, show a dialog. (use code similar to Mox Diamond for this part)
-The dialog should have the options 'Pay 2' and 'Sacrifice this'.
-Like Mox Diamond, if 'sacrifice' was chosen, or the dialog was not shown, bury the card. Otherwise charge 2 mana.
-Voila, you have Mulldrifter with the appropriate mana cost!
The same can be done for Shiekmaw, but not Cloudthresher, unless there is a way to reduce colored costs too.