How about an alternative evolution for Porygon2. Continue with the insanity of this Pokemon and require a new item and trade.
The idea here is that the Dubious Disc suggested once that the Porygon-Z evolution is not official Silph Co stuff. Think of Porygon-Z as a hack or unauthorized mod to Porygon2. Z can even be taken as a deformed 2.
Porygon3 would be a more official release and an alternate evolution. Just to mix things up how about giving it's unique move Sharpen (except it's on an Onix from a New York Event) some use and have Porygon3 have more emphasis on Attack.
Also fix Conversions to make them better and more useful. :P