In this blog post, I share my thoughts on what kind of game Torotoro Resistance may be making after they release the third part of Paradox.
The MGQ series is known for having a decent plot relative to other games in the genre, so this is something that would receive much attention.
The original trilogy is a straightforward battle to save the world and bring about peace. Paradox takes this further, featuring a battle to save the multiverse. There's really no way to continue the setting beyond that, given that by the end our main cast will be as powerful as gods or more powerful still.
One idea would be a prequel. There's plenty of room in the history of the setting for a game or three. We have: the journey of Marcellus and his group (unique for being a group of normal humans who still managed to reach Hellgondo); Heinrich's fight against Black Alice; and the Great Monster Wars. Of course, this has the problem of the ending being already known to people who've played the previous games. This could be solved by giving the option to choose an alternate route (e.g. Marcellus + co. refusing to kill Alice XV) and see how this would have changed history.
EDIT: Another idea would be an alternate universe with a completely different kind of setting. The modern day is an interesting possibility, which was previously discussed in this blog post: https://monstergirlquest.wikia.com/wiki/User_blog:ArmInAss/Modern-day_MGQ.
The original trilogy is a visual novel focused on a small group of characters, with Luka being the sole controllable character in the vast majority of fights. This means that these few characters are explored in depth, but limits interactivity. Paradox lets you recruit anyone and everyone and assemble literally any party imaginable. This lets you play the game pretty much however you want, but reduces the time that each character receives.
My idea is that the next game would be somewhere in the middle. It would focus on a specific group of characters, with a minimum of four (the size of Marcellus' group) and a maximum of twelve (the current maximum in Paradox).
EDIT: I've been on a Persona run lately, so I think one option would be to follow the style of this series: in addition to exploring dungeons and fighting enemies, you also build up your relationships with your party members via social interactions. Alternatively, they could remove the battle part altogether and go for a dating sim.