I used to be a Roguelike developer (and I mean that before the term was co-opted by people who never played Rogue/Moria/Nethack/Angband who are using the term wrongly); roguelikes are turn-based tile-based games, somewhat like Disgaea but usually with just one character.

One of the challenges with adapting this to multiplayer is handling how time works; in a single player game (like ZAngband or Disgaea 4, for example), it doesn't matter if you take a long time to plan out your turns because nobody else is waiting. This gets boring really quickly if someone else is waiting and you both need to end your turn before the next one starts.

There was one Angband variant I remember that tried it - MAngband; it was multiplayer and had a timeout for turns. It kinda-sorta worked, but people still struggled to adapt to the way time worked in it - it's just not that fun to wait so long in a game series that normally never has you waiting on anyone else. It's not clear whether some other idea would need to be found to manage this.

I suppose if the mechanics of combat were actually handled through the automation system introduced in Disgaea 6, that might work. I don't know if people would want to play Disgaea that way though.

You'd have two ways of going about it that wouldn't suck for an MMO and neither of it involves the standard disgaea turn based formula.

-Players are in control of a "generic" class with level ups, gear, skill trees and whatever. In this approach I'd imagine the game plays or moves kind of like... maybe Ragnarok or Dofus, maybe even translate to an ARPG kind of formula, which could really work.

-Players are in control of a fledgling netherworld, their leader being a customizable and souped up version of a base generic class. The MMO level track sees them upgrading facilities and capabilities to compete with other netherworlds, pirates, and what have you. Maybe an option to operate on a nethership (much like Disgaea 5, and 7), a good bit of the game itself would involve players simply trying to gain full control over their netherworld (much like Laharl and Zetta's respective journeys to establish control over their own).

Seasonal stories and gameplay mutators could be celestian forces meddling as usual, maybe throwback to the D1 earth forces invasion, Baal being an ♥♥♥♥♥♥♥, land of carnage expanding and destroying things, and then you can toss out big in-universe events, like void dark. Also because it'd be a mmo, you'd have to do something like a red moon event every 3 months for the prinnies and the general upheaval that causes.

You mean Disgaea RPG?

And yes, it did go as people who know the publishers expected thre game to go

Probably something more akin to Dofus/Wakfu

Dofus