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.