>>31571548>Fire/GroundGives it access to a nice ground STAB (let it have earth power for fuck's sake), removes stealth rock weakness, and is not that a x4 water weakness is too bad on this already fragile bastard
>Water/DarkIt's a vicious predator that tears prey apart acording to the pokedex, and that sounds more dark than dragon to me. I think crunch STAB would benefit it better, too. Give it access to knock off and a signature water fang move that can benefit from sheer force (I really wish its HA was strong jaw however, but mega sharpedo is a thing so dunno)
>Grass/FairyI would have said grass/psychic prior to fairies being a thing, but that's a godawful type combo. For a grass/fairy meganium, it would have to get some ACTUAL fairy moves, and it is not like it can't: flower shield, aromatic mist, dazzling gleam, floral healing, misty terrain, play rough, even fucking fleur cannon would fit it.