NM tend to release characters (and even uniforms) in accordance to what is going on in the comics at the time (e.g. ANAD stuff), MCU releases (i.e. movies and TV shows), or events being pushed by Marvel Games (e.g. Captain America's 75th Anniversary). Let's have a look at what's going on across various Marvel media.
Here's the current MCU release schedule for the rest of 2016:
>Agents of S.H.I.E.L.D. season 4 (Sep 20)
>Luke Cage (Sep 30)
>Doctor Strange (UK: Oct 28; USA: Nov 4)
Comic events:
>Civil War II (begun; delayed/extended; ending in late 2016)
>The Clone Conspiracy (Spider-Man event) (beginning in Oct)
>Marvel NOW! (2.0) (beginning in Oct)
>Inhumans vs. X-Men (beginning in Dec)
Obviously we won't be getting X-Men-related characters, but NM could use the Inhumans vs. X-Men event to release an Inhumans update. Possible characters being Medusa, Karnak (recently added in CoC), Triton, Crystal, Gorgon, or even some NuHumans (Iso, Inferno, Flint, Grid, Reader).
A CW II event tie-in could allow for an Ultimates update (America Chavez, Blue Marvel, Spectrum; Carol and T'Challa are already in the game).
With Marvel NOW! 2.0, 'Champions' is releasing (a solo book featuring a team of young superheroes) in which Sam Alexander (Nova) is starring.
Tinfoil hat theory time: in July, Marvel Tsum Tsum had an update that added Songbird, Squirrel Girl and White Tiger, and a month later 2.4.0 added those characters in a New Avengers-themed update. This month, Marvel Tsum Tsum added Kate Bishop and Jessica Drew. Does this mean those 2 characters are on their way to MFF?... Probably not.
It should also be noted that Carnage, Hyperion and Moon Knight all had ongoings at the time they were released in the game.
But at the end of the day, CMNN has stated NM don't have to tie-in with stuff like ANAD, and it's clear with the recent 'classic' Nebula uniform and other various uniforms (e.g. Ronan, Ms. Marvel).