All characters who have appeared in the NetherRealm era are included by default. The reason the character pool is limited to modern-era characters is because many of the 3D era fighters are obscure or niche, which could make the game significantly more difficult for players unfamiliar with the Midway era.
However, you can start a new game with 3D era characters in the pool by clicking the 3D Mode button. This will begin a new game that includes those characters.
What character lore is used?
In Mortal Kombat, a character’s traits and story often change from game to game depending on the current narrative. This means characters may acquire new attributes or roles over time.
To keep things consistent, this game uses the state of each character as they were before the creation of the New Era in Mortal Kombat 11 (2019). For example, Liu Kang is considered both a god and a human, due to the events in Mortal Kombat 11. If a character has had multiple traits due to story progression, they are reflected in this game. For instance, Scorpion starts as a revenant in Mortal Kombat (2011), but by the end of Mortal Kombat X, he is human again — so he is considered both "Human" and "Revenant" in this game.
Retcons are also treated as canon where applicable. So for example, Sindel is considered "Evil" rather than "Good, Unwillingly Evil".
What about the characters in Mortal Kombat 1?
3D era characters who only appear in the Mortal Kombat 1 (New Era) timeline are not included in the normal mode character pool.
I disagree with how a character's canon is established! Is there any way to get it fixed?
Feel free to contact me on X at Ailu_dev on X or on ailuinteractive@gmail.com, if you notice any mistakes or have feedback on how certain character traits should be represented. I'm open to revisiting decisions if there's a flaw in the logic.