You must not be familiar with the Shining series then, because those were some great rpgs, that had good writing. They made 10 total Shining games, including the Sega CD game and the Game Gear spin-offs, and most of them if not all of them were very good. They also made Beyond the Beyond on PS1. So Golden Sun is hardly their first go-round as an rpg maker, and that was precisely what they were known for, BEFORE they developed the first Hot Shots Golf on PS1, which is what lead to Nintendo hiring them to make Mario Golf, etc.
Golden Sun is okay, and a fourth game would be cool. But I think what would have more value, is if, since Nintendo is pretty cozy with Sega these days, they would iron out a deal with Sega for Camelot to make a new Shining Force (or other Shining) game, either for Wii U or 3DS (or hell, both). Nintendo could certainly use another exclusive game, and I think at this juncture there would likely be a lot more hype for a new Shining Force than there would be for another GS.