Havik its among the most favorite 3D characters of all time, he has a good chances specially story wise since he was behind the plot of blood magik and even manipulated Reiko to do his bidding.
Sareena is an extremely popular character, but NRS its kinda like a mad mom, Sareena Story is attached to multiple characters, and it just doesn't do justice if you can bring Sareena without bringing back the characters who are strongly connected to her story:
a) Bi-Han (Noob Saibot) and Kuai Liang (Sub-Zero)
b) Jataaka and Sareena (Sisterhood of Shadows) and of course their leader Quan Chi
So bringing back Sareena NRS had to bring those 5 too, besides Sareena its only more popular than Kia and Jataaka because at one time in story they decided to make her playable and she was lucky enough to be OP as hell so it kinda helped boosting up her popularity, but personally Sareena Story without 5 characters it feels a bit incomplete for several reasons.
Bo' Rai Cho had a good return despite the fact that i think his variations didn't do him much justice, but he was still had a good cameback
Hotaru cannot coexist on a MK Story without Darrius and Dairou and this goes back and forth with any of them, specially because their story intercalates and last time we knew about them, all their story was still interconnected.
We've seen what the Kenshi Story led to, if he is in this game, so will be Takeda and so will be at least one Red Dragon member to give plot or reason to his existence, otherwise i think its contradictory
Remember Li Mei has an appearance in MKX but there was nothing more attached to her story so she wasn't playable, not sure how different its going to be this time.
And then again, if a character its put in the game just for being there it will start to look weird because then their role in the game, story fits no purpose at all
Just take a look at MKX, character which were in the Story but had no major influence over it at all:
Reptile
Ermac
Ferra/Torr
Erron Black
Literally MKX would do fine without these story wise and you wouldn't even miss them at all.
Basically Ferra/Torr were just in because NRS wanted to try something new, but there wasn't like a real good spot plotwise for Ferra/Torr in the main story, same goes with Reptile and Ermac who were in the game due popularity demand. Erron Black had some role in the beggining that was it, for an acomplished Hunter who hunted a lot of species overtime he did very little in the game story.
But Erron Black had an amazing run in the comic which is where i think he made most of the impact.
Other characters who had a major impact in the story that were just NPC and weren't really there
Skarlet - Relative to the blood magik saga
Reiko - Same as above and Mileena's plot
Rain - Alligiance with Tania
I'm getting carried way, anyway we had to make an educated guess on who is on the roster its kinda difficult now specially because timelines can mean anything, we only know of 2 time lines but we dunno if this game will relate to classic and current timeline only or if its going to have alternatives