Same, either her or Wanda/Lorna.
I doubt it's Wanda, she was never a member of the X-men and at the time within the game, she'd be a member of the Avengers and the game will be dealing strictly with characters from the X-men cast. The devs have said that they're adding characters in order of their introduction.
And before anyone says "
bUt RoGuE aNd LaUrA wErE aDdEd FiRsT!"
They were proof of concept for what was originally going to be a mod. The devs weren't going to waste those assets. And arguably speaking, Rogue and Laura are amongst two of the most popular members of the team at this time.
If we look at the female members of the X-men as they were introduced in order:
1 - Jean Grey (The X-men #1, Sept 1963)
2 - Lorna Dane (The X-men #60, September 1969)
3 - Ororo Munroe (Giant Size X-men #1, July 1975)
4 - Kitty Pryde (X-men #138, October 1980)
5 - Anna Marie LeBeau (Uncanny X-men #171, July 1983)
6 - Betsy Braddock (Uncanny X-men #213, January 1987)
7 - Alison Blaire (Uncanny X-men #214, February 1987)
8 - Jubilation Lee (Uncanny X-men #258, February 1990)
And that takes us up to around the time of the Onslaught Saga, which is where the plot of the game will end. With Laura, that would take us to 9 characters. So that's approximately half the potential love interest. And before says "Ah, but that could mean X, Y or Z could appear!", understand that there have been multiple teams over the years that weren't named "X-men".
There were the New Mutants, of whom there was;
Danielle Moonstar, aka Mirage
Amara Aquilla, aka Magma
Illyana Rasputin, aka Magik
Rahne Sinclair, aka Wolfsbane
All of whom could be potential lovers for Null. (And yes, I am aware of Karma, however she is firmly a lesbian and as discussed on the discord, making her or any other lesbian characters bi for the sake of it, feels wrong).
There's Theresa Rourke Cassidy, aka Siryn.
There's also Emma Frost, aka the White Queen.
All of whom take us up to 15 characters for a 6 chapter game. And if there to be 18 characters required, there would be other characters more closely affiliated to the X-men to use than dipping into the Avengers.