Darunia should be the new Zelda character Gorons rule.
And he suffers the same problem every non-Triforce Trio character faces; lack of appearances. Darunia is by far the most important Goron and most iconic Goron to appear in the series, and Gorons are the most recurring non-Hylian race, but no single Goron plays that important of a role in any one Zelda game, with Darunia's role being the biggest (and even then, its not that notable).
I mean, yeah, Darunia's rolling attacks could make for a decent moveset, but its not that unique on its own merit, and for a relatively minor character in the grand scheme of things in the Zelda series, there's not much going for him. A one-off character would probably have to be incredibly important and have plenty of original material to work with.
I absolutely love the idea of a Sage (yes, technically Zelda/Sheik is one in OOT, but Zelda tends not to get treated like a Sage otherwise, so I mean "other than Zelda"). Not many of them have a lot of material to work with overall, though. Darunia is not shown using a hammer in the series, not that it matters much because he intended story-wise to use the Megaton Hammer in OOT, so that could be justified. But, the hammer isn't anything new, and other than the Hammer he'd basically just be a heavyweight Sonic. He doesn't have much material for a recovery move either.
The most original pick for Sages would probably be Saria (and she's the most important non-Zelda Sage in OOT) if they go with idea of using nature-based moves and an instrument for a weapon (instruments being something not only not represented in Smash, but an extremely iconic Zelda trait), but there's a fat chance of that ever happening.
I'd totally support it, though again, I see no chance of it ever happening.