First of all, I'm not suggesting identical gameplay to SSB, just the presence of various characters from various series in a MK-styled game. I'm pretty sure WB would have the money and resources for such a game.
NRS has done a great job with most of the guests and DC characters imo, and I'd like to see them take a shot with a lot more non-MK characters in a crossover fighting game. Guests are fun, but we can only get an handful of them in a MK game.
Anyway, some picks of mine for a roster:
MK characters other than. the inevitably-present Scorpion and Sub-Zero. A lot to choose from but I'll make one suggestion: Baraka.
All the guests we've had in the MK series, though I could do without Kratos. And Freddy needs Robert Englund's likeness and voice.
Terminator: The classic T-800, the T-1000 (with both Robert Patrick and Bryung-hun Lee from Genisys as selectable designs), the TX and John Connor with Skynet as his alt and a Linda Hamilton skin for Sonya.
Robocop, with both the classic Peter Weller version and the one from the Remake as selectable designs.
Various horror characters like Pinhead, Jack Torrance (from the Shining), Ash, and Michael Myers.
Dexter Morgan
Isaac Clarke (Dead Space)
Dante and Vergil from Devil May Cry
Resident Evil characters such as Jill Valentine, Ada Wong and Albert Wesker, and monsters such as Tyrant, Nemesis and Ustanak.
Pyramid Head from Silent Hill.
PS4 exclusives: Sweet Tooth, Mael Radec (Killzone), the Bloodborne protagonist, Kratos, Zeus.
XB1 exclusives: Master Chief, Arbiter, Marcus Fenix with every other Gears as alts, Locusts like Raam, Killer Instinct characters.
Venom and Carnage
Ghost Rider
That's it for now.