I would rather they kept their already existing characters and improved on the less popular ones. I remember Stryker being one of the most hated characters of all times, but his appearance in the new Mortal Kombat was awesome. He looked good, had some cool moves and a nice spot in the storyplot!
I don't want any new characters in the next Mortal Kombat, though Skarlet was kinda cool. I do wonder what will happen, since MK4 mainly happened in Edenia and Edenia isn't freed in the new timeline. Also the Deadly Alliance can't happen since Shang Tsung is no more. I think MK10 should end with the awakening of the Dragon King as a cliffhanger to the next game.
But first, let's see who is alive from the first era:
Raiden
Sonya
Cage
Cyrax
Sektor
Scorpion
Quan Chi
Kano
Mileena
Baraka
Reptile
Sheeva
Ermac
Rain
Skarlet
Then there are those who are dead and under Quan Chi's control:
Kung Lao
sub-Zero
Sindel
Kitana
Jade
Nightwolf
Smoke
Jax
Stryker
Kabal
And those we don't know what happened to, but appears to be gone:
Liu Kang
Shang Tsung
Noob Saibot
Those were the characters of the first era, then there are the characters from MK4:
Kai
Reiko
Jarek
Tanya
Fujin
Shinnok
Shinnok will most likely be the boss. Quan Chi will most likely use the dead fighters under his control as part of the invasion army, so that puts the Earthrealm defenders on the losing side, as they are WAY fewer than the bad guys. The surviving bad guys from the first era are kinda wild cards though as we don't know if they will support Shinnok or go against him or maybe even just stay neutral and do nothing, though I doubt that will happen.
I would also like for the MK Deadly Alliance and MK Deception characters to be part of it, but I'm not sure how they will be part of it all. Hopefully Netherrealm Studios come up with a good storyline which will fit them all.
Just for good measures, I'll mention the MK Deadly Alliance and MK Deception characters too:
Bo'Rai Cho
Li Mei
Kenshi
Mavado
Hsu Hao
Nitara
Drahmin
Moloch
Mokap
Sareena (appeared in the handheld version called MK Tournament Edition)
and
Ashrah
Hotaru
Dairou
Darrius
Havik
Kira
Kobra
Shujinko
And of course the Dragon King, Onaga.
The only characters not mentioned are Taven, Daegon and Blaze which are only important in Armageddon. However, Raiden has tried to prevent Armageddon, so if he succeeds, the story with these three characters will be completely different. I can say one thing for sure, the next Mortal Kombat will be alot different than it was in the past timeline.