What would be the awesomest thing ever in crossovers? I would totally cream my jeans if SMG and Eliza showed up at the end of 'Supernatural' with no dialog... just standing in a cemetery looking at Sam and Dean with stakes and an eyebrow in the air... leaving it completely ambiguous and yet obvious what the intent is....
Can you imagine the fan-fic that would take off from that?!
And, it would avoid the contractual/patent issues, because hiring SMG and Dushku for a part isn't illegal, and since they have no dialog, who's to say that they are Buffy or Faith?
That would be absolutely awesome as an ambiguous ending.