Just a huge book with all things MK, it could give us character bios, details, it could dig into the lore, such as things with the One Being, The Elder Gods, the creation of the tournament, and go all the way to where MKX ends.

It could also have a bunch of cool other things, it could have an exact timeline as to which everything takes place, it give us detailed pictures to what all the realms look like from a birds eye view (Imagine looking at Outworld or the Netherealm on google earth lol) or imagine being able to look and be like "oh so this is where the Wasteland is, and the Deadpool is way over here, and oh look there is the Living Forest, its massive!" it could have character statistics, relationships, random facts about the characters, from creative a standpoint and some in game facts too , pretty much everything.

I know a lot if not most if the things I listed can be found with research, especially within the games, on the MK Wikia and the regular one but I think an official MK Encyclopedia would be really cool to have.