Orianna's not really an assassin but you are short on mages. I'd recommend her because I'm on an Orianna kick, but she's super hard to play properly, be warned. A simpler mage that's still quite powerful is Morgana.
She's not, but I found that she can still do an impressive amount of burst damage to single targets if she can chain her abilities properly. I'm using "assassin" here to mean a champ that's built to take on single targets with little in the way of area damage. The opposite of "pusher", if you will.
She's also 1500 IP cheaper. There's also the fact that she's far more uncommon so people are less well-versed in how to play to counter her. Everybody knows Morgana. I'm actually kind partial to obscure champs just because it leaves people a bit off-balance. Plus, I get less "everybody knows with that champ you buy A, B, C, D and *then* E! fukin n00b lrn2play" 
You want an obscure champ? Get Karma. She's also a mage to boot!
This.
Nobody plays Karma ---> Nobody sees Karma
Nobody sees Karma ---> Nobody plays against Karma
Nobody plays against Karma ---> Nobody knows how to counter Karma
Nobody knows how to counter Karma ---> Nobody beats you when you play Karma
Nobody beats you when you play Karma ---> Infinite IP and endless amount of victories.