Old-school gamers might recall how you technically could get Cloud Strife to Level 99 on the PS1 version of Final Fantasy 7, but you weren't really supposed to do that. The gameplay just wasn't built around that type of grind, which got old — fast.
Alternatively, Disgaea is a series that's explicitly built around that exact hustle, and the level cap isn't 99: it's 9,999. Yep, you read that right.
Aside from a ludicrously-high character level, every item you pick up in Disgaea has its own randomized dungeon, all so you can level up said item to 9,999. Theoretically speaking, there's no cap to the number of hours you could spend here. Some have certainly spent thousands upon thousands ...
Thankfully, the series' strategy RPG combat stays fun during the endless grind, and all of the Disgaea titles feature tongue-in-cheek characters and interactions to keep things entertaining.
I'm a fan of Disgaea 2's PC port, but honestly, any of these titles on either console or PC are just phenomenal and worth sinking time into. Want the latest and greatest? Disgaea 5 is the most recent main entry to hit PS4.