How much challenge you lookin' for?
A Little?
Try Johnny or Scorpion (viable and still strong but with weakness)
A Moderate Amount?
Try Kano or Kabal (good but risky)
Hard Mode?
Try Kotal or Shao (very heavily flawed and hard to play)
Wanna Torture Yourself?
Try D'Vorah (you will lose, and lose, and lose)
What this guy said^
Kotal and Shao are so flawed you will be in practice every day trying to make bad design viable same with D'Vorah.
Love the game so don't get me wrong but none of these characters in MK11 have depth like MKX. Erron Black and Sonya have some hard inputs but very basic movesets. Erron has the following hard dial strings: 21212 ( can slide 2121-2 or input as 21-1-1+2) then he has F2121, B122, B2U4222 but other than that he is basic other than having Locked N Loaded gun stance in his 52 Card Pickup Tvariation.
Kabal is extremely easy to use.
Jacqui has cancels and gimicks to mess around with.
But Scorpion, SubZero, Sonya, Kitana, Kollector, Scarlet, Jade, are very bare bones.
I hope they add depth to certain Characters because I was like you in mkx, I used predator with his cancels when I got board of basic characters and used Cyrax of course with all of his traps and gimicky combo starters. I also like characters with more depth.
Kotal is actually fun just sucks right now but there is more to him than say Scorpion.