I'd like them to just work with the current types anyway. Create interesting and unique dual types that aren't just exclusive to legendaries. I really hate how all the experimentation in dual typing has only been on the new legendaries in the previous games (or at least, the dual types that would be cool to have on regular Pokemon).
They could do crazy stuff like Steel/Poison or Fire/Psychic (I'm aware of Darminatan's other form). Or stuff like Water/Poison. Dual types of lesser seen or used types... you know, so there's more prevalence of those moves in battles.
How cool would it be to see something like Ice/Psychic, Ice/Poison, or Ice/Bug?!
Note: I'm not thinking of weaknesses when talking about this, just the idea of more variety of dual types than the same ones we've been getting since Gen 2.