Which Restricted Legend/Form Type Combos Should be Shared?

Pokémon come in all sorts of typings, with some of those combinations being unique from any other, and that can be cool on an early route Pokémon that you can pick up for the whole game, but it's a lot more annoying to find an exclusive type pairing on a Legendary or even rare form change. So let's look at some of the worst one-offs and see which type combinations need to be set free from their shackles and finally given to the common Pokémon!