Well being super effective with each other isn't exactly a bad thing... it'd be like ghost/normal, none affect each other.
Being super effective on each other can be a huge gamble... like dragons fighting dragons (but you don't do that, because that's dumb...)
A new type doesn't sound wise; the types already create enough strategic game play as it is, a new one may upset the balance of the game... we can think of new dual types, but a brand new type wouldn't be all that great... I still don't understand why they made Curse a ??? type... that just makes things confusing... they could've easily made it a Dark move or a Ghost move... or make them separate moves? Confuzzling...
Back on the ??? type however: probably a type that takes neutral hits from any attack, much like a dark/ghost combination... but at the same time, you won't receive any bonuses from any attacks... Judgement would probably become ??? type, that hits everything for neutral damage... that would be amazing type coverage (and sounds most possible)