I honestly pictured Cage as more like Yamcha.

Shao Kahn - Frieza & Onaga - Majin Buu.

There is a huge difference between the characters you mentioned.
Liu's human and understands things a lot more readily than Goku.
Rain didn't know he was a prince/Demi-god.
Krillin's not really a comic relief/suave pompous guy like Cage.
Shao Kahn doesn't buy and sell worlds, he conquers them to keep for his own.
Scorpion's very selfish in his pursuits, and the younger Sub-Zero was hardly a bad guy when he debuted.

The difference between the teams of....

Forces of Light vs Z Fighters

- Forces of Light
Liu Kang
Kung Lao
Johnny Cage
Sonya Blade
Kitana
Sub-Zero
Jax Briggs
Nightwolf
Kabal
Kurtis Stryker
Cyrax
Ermac
Jade
Smoke
Kenshi
Kai
Bo'Rai Cho
Raiden
Fujin

- Z Fighters
Goku
Gohan
Vegeta
Trunks
Goten
Piccolo
Krillin
Tien
Yamcha
Chiaotzu
Uub/Majuub
Android 18

- Forces of Darkness
Shang Tsung
Quan Chi
Kano
Scorpion
Mileena
Baraka
Reptile
Sektor
Sindel
Noob Saibot
Rain
Skarlet
Shao Kahn
Goro
Kintaro

- Dark Fighters
Frieza
Cell
Majin Buu (Kid)
Garlic Jr
Turles
Lord Slug
Cooler
Broly
Bojack
Janemba
Baby
Super Android 17
Nappa
Raditz

I think it's safe to say that Goku could probably take on EVERY single MK character on his own at the same time, and I'm not just saying that because I'm a long time DBZ fan. There's really no competition between the two. But, if I were to compare them with MK characters, it would go like this:
Liu Kang - Goku ... Only because of their status. Both are practically undefeated and are the main protector of their worlds.
Kung Lao - Krillin ... DO NOT ATTACK ME! I only compared these two because they are best friends of the main protectors. Other than that they have NOTHING in common.
Freeza/Cell/Buu - Shao Kahn ... I compare Freeza and Kahn because they conquer worlds. Freeza sells them but Kahn takes them for his own rule (as said above). Cell and Kahn, well... They like tournaments? The Cell Games and Mortal Kombat obviously. Buu and Kahn... THEY'RE BOTH CRAZY! They like to blow sh*t up and cause complete mayhem. They also love watching people suffer. Only difference is Kahn is probably smarter.
Scorpion - Vegeta ... Although Scorpion isn't a Prince, both he and Vegeta were pretty neutral characters. Scorpion fought along side anyone who could help him and Vegeta fought any enemies that came after Goku only because HE wanted to be the one to kill him. Both Scorpion and Vegeta also have pretty aggressive and intimidating attitudes.
Johnny Cage - Yamcha ... Come on... Do I REALLY need to get into this one lol? They are one in the same in two different universes.
Sub-Zero (Kuai Liang) - Tien ... They both have that "I'm a good guy, but I won't hesitate to kick your ass if I have to" attitude. They both seem like they're always looking for ways to be a better fighter. Subbie just doesn't have a third eye xD.
I think I could go all day with this but I'll stop there. This was a fun little topic

But Rain's a prince, so he's like Vegeta

As for Scorpion, he could be the Piccolo of the series.

If Liu and Kitana had a son, he'd be either like Gohan or Trunks.

The son could be a nephew to Rain. Trunks and Vegeta are father and son, while the other is the future nephew of Rain