As you've already said, Nasty Plot and Destiny Bond are basically required egg moves. However, seeing as ol' 'Magius only has a base 60 Atk stat, Shadow Sneak and Sucker Punch won't be doing all that much damage to opponents, considering its Sp. Atk is 105. They are still good choices, and you should definitely choose one of the two if you can't decide on a fourth egg move.
I'd probably stay away from Curse altogether, unless you're not a ghost type. Imprison is pretty niche, although might have some use in the Halloween competition, so it might have some use. Memento is pretty decent as Mismagius is relatively fast, the only problem being that your opponent can just switch out the turn after... Spite is a level-up move, so there is no reason to breed the thing on when you can just relearn the move at the cost of a Heart Scale.
Me First is also an option, and would go nicely with its 105 base Speed and a Substitute, should you decide to forego Sucker Punch (/Shadow Sneak).
If you're looking for gimmicky moves or niche moves, then the other egg moves (Ominous Wind, Screech, Skill Swap, and Wonder Room) might find some use, depending on how unconventional you want your Mismagius.