A trio based off Loki's kids: Hel, Jormungandr, and Fenrir. Maybe a fourth (like Keldeo to the Legendary Swords) based on Sleipnir.
Either this or no trio at all. I've kind of been turned off legendaries for a while now because it's just the same thing generation after generation, and a fair chunk each time just become event based or exclusive anyway. The whole trio thing in particular is getting pretty old, especially from gen 3 onward. Hoenn had two trios, Sinnoh had two trios, Unova had three trios, it's getting a bit excessive.