Well, no joke characters (Mokrap, Meat) and no more ninjas (I love them as much as the next person does, but we have enough), that I know for sure.
For males-
Male Lycanthrope of some sort (Werewolf, Weretiger, Werelion, Werebear, generally easy to access for new players since he is both strong and powerful)
Gladiator style Roman or Greek warrior that concentrates on using a shield and either short sword (Gladiator) or spear (Hoplite/Phalanx) that is highly defensive and focuses on counters (for more advanced players)
Ronin/Samurai (yes we have Hotaru and Kenshi, but I'd like to see a dual wielding type that takes a lot of skill, would be even cooler if he was from Outworld or non-human)
For Females
Sorceress/Witch/Seer (with all the magic going on I'm surprised we haven't had one as of yet aside from Taven/Daegon's mother, probably a good witch that opposes Quan Chi and Shang Tsung, possibly an older, wise, experienced woman, like a Japanese miko or Korean/Chinese shamaness)
A Powerhouse female character that is not scantily clad, ultra-thin, or looks like a cover model (not that I mind the MK ladies, just going for something different), maybe a grappler based character that is more agile, like a female equivalent of Jax (but human or Edenian, not a Shokan like Sheeva)
A Cybernetically enhanced female of some sort, either a power hungry regular human inside a tech suit (C. Viper, Lex Luthor), a female Cyborg (fembot version of Cyrax and Sektor) or an android that looks like a regular woman (like the fembot in Terminator).
They're just ideas...so take what you will of them lol.