Actually I just want to see various characters dance. Suikoden has so many different cultures, music and dancing is a great way to express/display them. The dancing Kobolds in S2 wa a good idea (it actually looked koboldish), but I never understood why they were dancing to the winger theme instead of the kobold theme. Anyway, the songs for dancing should be exclusive dance songs, not a theme you already got sick of, and it should be in the style/culture of the performers.
Oh yeah, humanoid races and tribal characters could have some interesting ritual or celebration dances. And knightly/noble/royalty class characters would likely have some ball dancing experience. The towns always do have their own anthems, so the music could include different renditions of them.
Maybe it doesn't even have to be a dancing sub game, but just something that occurs when you access the dance floor with certain characters in your group, much like the bath scenes. there can be some graceful elegant dances between some characters, and goofy dances with others.
Castle designing could be like the Hearthfire content on Skyrim, where you decide which room goes where, and you'd need access to materials to build each room. As it's Suikoden, then you'd have to choose your rooms carefully, depending on which characters you've recruited. So if you've recruited a blacksmith, you won't be able to have him work on your weapons until you've built a smithy for him to work in. And if you build a diner early, you won't be able to start making profits from it until you've recruited a chef, that sort of thing.
Banner designing is a nice idea. The game could provide different elements to add to a banner of your choice, but another nice touch would be, with the way gaming technology has advanced, we could import our own imagery to be used in the game. Maybe when we've recruited an artist like Ivanov or Jude, and built an art gallery in our HQ, we could access an online gallery that lets us view other players' artwork and buy images from each other! I don't know if there are any modern games that currently allow this sort of thing.
Of course, this is going into network gaming territory, which is all good I suppose, but I'm thinking more about gaming features related to the storyline and where we can have fun with the characters.