(last updated: 12/28/17)
The challenges associated with Mortal Kombat have a lot to do with Outworld, and the monstrous creatures that reside in it. In the world as we know it, these creatures were all human, and were all parts of Earth at some point. Even the four-armed Shokan race were said to be tainted by dragon's blood--whatever a dragon might actually have been in those ancient times.
Outworld itself is no longer a specific dimension, but a metanatural area to which the leylines naturally extend. Outworld's presence as an interplanar dominator has become a matter of the afterlife itself--Outworld seeks to control both the natural and beyond-natural worlds with its host of monstrous denizens, but is largely held back by the fact that it has not been allowed by some of the strongest forces in the afterlife, some of which are literal gods. Characters from Outworld may be fish out of water, summoned for specific reasons by the sorcerers of Outworld.
It also remains to be said that characters from Mortal Kombat cannot wantonly tear eachother limb from limb, no matter how low the other person's health is. Nor can they tear people not from Mortal Kombat limb from limb. Tearing people limb from limb is an act that disrupts casual game play.
As a rule, only characters that are playable from the first three main Mortal Kombat titles are acceptable, in addition to the rebooted Mortal Kombat from 2011 and Mortal Kombat X. This is due to the large number of thinly fleshed out and joke characters available in many of the other side games and later entries to the franchise.
Because the storyline for MotM is just starting, many of the characters from Mortal Kombat X have been modified as family members of existing FCs as opposed to being children, so as not to dictate canon to other FCs. Outworld has not developed a peace accord with Earth, and whether or not the Cyber Initiative has yet modified Cyrax, Sektor, etc. is in the hands of the individual players apping them.
Several characters are not allowed from the game Mortal Kombat. SHAO KAHN is a plot character, and due to using elements that the combat system cannot support, SHANG TSUNG and QUAN CHI are also unable to be apped. TAKEDA, due to being completely dependent on another character's storyline with changes unable to be made without compromising the character is also not appable.