For Jacquie in particular, I think there are a lot of ways they could go with her to make her much cooler than she was in MKX. I mean, check that original concept art and think about what they could do with that and the gear system!
Then they should design a new character altogether. Jacqui was beyond saving, her redesign would have to be so radically different that she may as well be an entirely new character.
Almost every character can be summed up in a few lines that encompasses what their special skills or abilities are, and they should become instantly recognizable.
Example:
He's a ninja, uses a kunai, and is able to fight with the powers of hellfire. You know it's Scorpion.
He's big and bulky, military and has cybernetic/metal arms which he uses to fight with. Jax of course.
He's a sorcerer, one who is very adept at stealing souls and absorbing their power, effectively morphing into other fighters. Shang, right?
Now what does Jacqui have?
She's a female soldier. Sonya? Nope, she's young. And she fires guns. Cassie? No. No, she's black too.
OOOOOH YOU MEANT JACQUI.
She has nothing going for her. Cassie has the wit. The other Kombat Kids all had something that made them unique: Takeda's whips, Kung Jin's staff/longbow, hell even Cassie offered more variety.
Jacqui should be an NPC at best. She only existed in MKX to justify Jax, and to force some romantic subplot with Takeda (wtf?)