Elder God Kenshi

Spec Ops/Strike Force, or as I call him now, Mall Ninja Scorpion.

The Day of the Dead exclusives had Kitana, Jade, & my personal favourite skin of Erron Black in the series

We'll probably never get these in the main games cause some LatinX faggots will start dilating & screeching "cultural appropriation".
There was also a Kold War exclusive line too.

There was once a time when you could even unlock some exclusive skins in MKX through the mobile games console link. There was even an exploit which removed the need for the mobile app to get the skins.
Ninja Mime Johnny Cage, which later came back for MK11

Farmer Jax from the MKX cutscene, if you paint him yellow, you can imagine fighting city slickers with this guy.

Cybernetic Jacqui

Undercover Cassie
Later for MK11 release we got the best male design in MKX, now at it's peak design. Black Dragon Tremor:

They gave the best design to one of the best MKX designs. I love MKX Tremor & I wish this how he is always adapted since MKX onwards.
Ironically, some of the ugliest skins also came out of this game. Some examples being:
Punk Rock Cassie Cage
The Kombat Cup Pack, which led to, Man-bun Johnny Cage, Karen bull-dyke Cassie, & an ugly Sonya with a neat hairdo
Now for that unfairness, as many know that MK11 butchered a lot of lore, one of which being Skarlet's lore about her blood absorption, retconned into some blood mage, all just to put her in a leather burkha with exposed feet (the character designer for MK11 is a massive footfag). This philosophy of theirs had the integrity of a wet tissue, considering they later made Sindel hot anyway, except NRS fucked that by making her into a power-digging cunt, & by proxy making it look like they're sending a message that "sexy = evil".
What many don't know is that the mobile game got a great skin for Skarlet, called Assassin Skarlet.

I'm not saying it's a the best Skarlet skin of all time, even if it is best Skarlet skin post-MK9, her hair still looks wacky to me, & I still prefer MK9 version over it just for that alone (I liked her ponytail) But if there's one skin that deserves to be in the main game, it's this one.