Honestly, as long as guests characters are still a thing, it's high time for a female guest. I think it's hard to fathom that they would go 4 straight games without one.
Im All The Way Up...But...Im Going Back to Sleep Now in a Pretty Place Where The Flowers Grow...i'll Be Back in an Hour or So.
Correct me if I am wrong but we nearly had a female guest character in MK11. I remember Ed Boon mentioning in an interview that I believe Lara Croft from the Tomb Raider series was considered but similar to Ash Williams they couldn't get the licensing right. But yes we came close in MK11.
Another female guest character mentioned for MK was Buffy the Vampire Slayer. She may have fit in well due to the television series' inclusion of her combat skills, arsenal, partnerships (especially with the witch Willow), and defeating of various creatures. The major difference would be having the human Buffy eviscerated, smashed, or incinerated. On the show, she had been stabbed/impaled, bitten, drowned, fatally drained by mystical energy, (in an alternate timeline) neck-snapped, and (her robotic counterpart only) decapitated or limbs pulled apart.