The portal for Etheria is a no-brainer. Everyone seems to favor that idea!
Here's two ideas I haven't seen:
1) Sculpt hinges on BOTH SIDES of the facade, and BOTH sides of the 'wall' side. Why, may you ask?

That would allow you, if you buy multiple castles, to add another wall to have a 3-walled castle Grayskull. (Or bigger)
Think of it as Mechaneck's neck attachments. You can add up if you have an extra neck part.
Granted, it would be the same wall (only reversed) but should Mattel ever want to sell us a Castle Grayskull extension, we could just connect the new extension part to that extra hinge on the side where there's no wall. That would keep the possibilities open! I think it makes a lot of sense.
2) Add a silver exclusive Power Sword! One of the main incentives of the 200X grayskull was for me that chromed sword that came with the castle. Sure, the castle was sweet too but I just wanted that bonus sword so much! It looked freaking cool in the Hall of secrets and you could see it through the window!
If Mattel could give us an exclusive vac metal Power Sword to ONLY the people who pre-order CC, this would also be a strong incentive to pre-order it! And a nod to the 200X castle Grayskull. Of course, what would be even BETTER would be a pewter metal sword. But that's probably too expensive.