This is very subjective. Like I love playing Sonya but if you like playing Ermac, I wouldn't suggest you even stare at her jumblies, much less play her.
My assessment of Ermac is that he's an easy mofo to do massive damage with from pratically any range. I think the only character you would be truly happy with as a sub is Shao Khan
Serious suggestions? Hmmm.... anyone in hat tier really. Raiden, Kung Lao, Freddy. Sektor has similarly-ranged tools to Ermac that you might find interesting.
As far as "checking a character out", it's gotta be at least an hour of serious gametime. Learning the specials, learning the basic strings and then using them against an Expert AI. Just picking up a character and saying "Sonya's bewbies are too big. Me no likee" isn't trying out a character.
If you're getting bored with that many characters, odds are it's not the game's fault.