So I recently finished FF XV a few weeks back, and while in my humble opinion, the Suikoden series is vastly superior in terms of storytelling and characterization, one thing FF XV did well was that it gave you quite a lot to do after the game finishes. There are several dungeons you can visit that have much stronger monsters that really motivate you to grind your characters further, and become the strongest they can be.
I think we can all agree that Suikoden, for all of its strong points, has never had too many difficult battles. Plus, a lot of players like to overpower their characters, decking them out with double-edged runes and sharpening their weapons to level 16 and whatnot. I wish there was a way to actually give these overpowered characters something to do by adding some kind of secret, mystical postgame dungeon (more Sindar ruins? why not!), one filled with monsters that could give these overpowered parties a run for their money.
Just me sharing a couple of my thoughts!