It could have had a moveset built around switching types to alter its own properties as well as those of its attacks. Kind of like Shulk's monado stuff, but instead of simply changing a stat like speed or weight, it could give Multi-Attack different on-hit effects like freezing as an Ice move or increased shield damage as a Psychic move, or it could alter a different stat, like using Ghost type to improve Silvally's frame data for dodging or even temporarily removing the penalty for repeated use of dodges while in Ghost mode.