I can't help but wonder why slippers are genetically incompatible. You would think that paphs and phrags would form viable hybrids, but they don't, from what I hear.
Maybe other slipper crosses are possible (Selenipedium x Mexipedium, Mexipedium x Cypripedium, etc).
Whoever manages to pull off these crosses will become famous in the orchid world, and I'll bet that the hybrids would cost a fortune.
Here are some crosses I think would look awesome:
Paph. malipoense x Cyp. japonicum (would this one be called a Cypripaphium?)
Paph. sanderianum x Phrag. caudatum
Cyp. reginae x Phrag. kovachii
Mexipedium xerophyticum x Cyp. calceolus (Cyprimexium or Mexicyprium LOL)
Cyp. calceolus x Phrag. besseae (Cypriphragmium/Phragmicyprium)
Cyp. parviflorum x Paph. armeniacum