I got some ideas for classic character fatalities that they should have used but did not and even characters that already have one I will post an idea for a 4th fatality bosses & dlc are included.
Scorpion 4th fatality spear slice from mk 2.
Sub-Zero 4th fatality ice grenade from mk 2.
Reptile 4th fatality Invisible Torso removal from mk 2
Ermac 3rd fatality telekinetic slam explosion
Smoke 3rd fatality invisible possession from mk 3 ultimate mk 3 and trilogy.
Cyrax 3rd fatality helicopter
Sektor 3rd fatality compactor crush
Johnny Cage 3rd fatality torso kick from mk trilogy n64 version or forceful shadow kick from snes version of mk 1 and shaolin monks
Rayden I prefer to spell his name that way because Raiden reminds me of a character from metal gear solid 2, 4, and the future game rising which is mgs 5.
3rd fatality Exploding shock
Kano 3rd fatality eye laser or skeleton rip whichever one you think  would be better
Shang Tsung 3rd fatality soul stealer
Sonya 3rd fatality fireball kiss
Kitana 3rd fatality kiss of death 
Mileena 3rd fatality man eater bonespitter
Jade 3rd fatality staff shake
Jax 3rd fatality giant stomp
Kung Lao 3rd fatality spin
Baraka 3rd fatality blade impale lift
Sindel 3rd fatality hair spin I thought that her scream skinner fatality would be better but I realized she already has a scream fatality in the game in which she screams in her opponents ear until half of there head explodes vertically.
Sheeva 3rd fatality head pound
Kabal 3rd fatality head inflation remember this fatality? he would use his respirator and inflates the opponent's head like a balloon. The opponent floats on top of the screen. An explosion would be heard as their blood, bones and organs rain down.
Stryker 3rd fatality explosives remember this one he would attach an explosive charge to his victims stomach jump out of the way and plug his ears before it explodes blowing the foe to pieces well in this version he takes out a takes a few steps back and takes out a detonator and presses a button on it to blow up the bomb and kill the oppponent and just like in his first two fatalites he would see it three times as a replay.
Nightwolf 3rd fatality lightning summon I was going to use his spiritual teleport but his second fatality is an update to that one.
Noob Saibot 3rd fatality teleport slam explosion
Liu Kang 3rd fatality arcade drop or cartwheel kick and uppercut but with blood splattering everywhere in this version
Quan Chi 3rd fatality neck stretch but in this version an x-ray animation is displayed showing what is happening inside to opponents body and organs you see the actual method of the fatality is not just to stretch out the losers neck and elongate it but rather completely dislocate there spine which would include several innards or insides being ripped apart resulting in loss of life aka death. the opponent would still stagger around with the giraffe like neck before falling down dead
Rain 3rd fatality when lightning strikes
Kenshi 3rd fatality telekinetic contortion from Deadly Alliance
Goro 3rd fatality dragon fangs from deception for gamecube and unchained for psp
Shao Kahn 3rd fatality headcrusher or double hammer pound from n64 version of trilogy he would pound the opponent into the ground with his hammer and laugh then bash them again and they explode to pieces.

incase you disagree with Quan Chi's neck stretch then his 3rd fatality should be his fatality steal from mk 4 however in this version he copies either one of the opponents fatalities and kills them with it at random he may even use there classic fatality against them For Example Scorpion's toasty breath except he does not rip off his mask well he does not even wear one he just breathes fire on scorpion burning him alive and removing his skin exposing his skeleton.
Or Reptiles tongue head rip and eat he just grabs his face with a long tounge and pulls his head off with it and devours it