Since CompIF did say they wanted to make a Nep game of every genre, maybe a fighting or rhythm game is next? Now they have JRPG, Hack 'n Slash, Shooter (kinda), Retro, VR, and a Mobile Game. Honestly, next might actually be a Gacha (which I wouldn't be mad with, I'd play it religiously) but in terms of a spin-off game, I'm really hoping that next, they make a fighting game. Here's what I think the base roster would be.
Goddesses (plus Uzume, Plutia, P-Ko)
Sisters (would be cool if Rom and Ram were like Hisui and Kohaku, both have their original characters but also have a dual character)
IF and Compa
The Gold Third
I wouldn't think adding more to this base roster would be a wise choice, it already has 19 characters (18 if Rom and Ram don't have a dual character or 17 if they only have a dual character) Then maybe Rei, CFWs, other Creators, and other spin-off characters as DLC. Honestly, you can replace the Gold Third with Dengeki and Famitsu. Either way the other can be DLC. Or if CompIF is really bold they could just put them both on the roster and have the base as 21 (which would never happen ever).
I think there's pretty obviously gonna be an awakening mechanic in the game (like Blazblue's overdrive). The mechanic can make all attacks cancel into supers and EX skills (except DP and grabs of course) and have passive meter gain, this should always last for 10 seconds. I think for that the goddesses, sisters, and the gold third could transform into their HDD/Gold forms (maybe Arfoire can transform, but idk if that would really work) and for characters like IF and Compa, they get a similar power boost but no change in gameplay.
Instant Kill should obviously be in the game, no fighting game is complete without them (cough cough GGStrive cough).
For something a bit extra and guessing CompIF's budget, will never happen, maybe the characters can get alternate costumes? For example, Blanc can have her Ultradimension costume and Megatagmension costumes to fight with (similar to Noel's NOL costume in Blazblue).
That's all I wanted to say about a Nep FG. Some of what I said will probably never happen. But I'd still be happy with a fighting game regardless of how they do it... Unless they make it like phantom breaker omnia, in which case I would definitely buy it but NEVER play it. But I have hope CompIF won't stoop so low.