I think Hilda won't happen, judging on the fact that their was no Midna alt for brawl. I feel as is SSB is very conservative when it comes to alt costumes so i'd be surprised if we even get a Skyward Sword Zelda alternate.
About other characters palette swap, we had some in the past. Mario have a Foreman Spike from Wrecking Crew and Wario outfit, Luigi have Waluigi, Peach have Daisy, there's Dark Link, OoT Zelda, OoT Ganondorf, Retro Link (on Toon Link), Olimar have Louis, there's Blood Falcon, Ike had the Greil outfit and Marth had the Leif outfit... Colors schemes ressembling others characters exist.
So I don't think a Hilda is out of the realm of possibilities. My only worry for Hilda (and OoT Zelda) is the blonde purple Zelda outfit.
About an SS Zelda 3D model, I don't think it's impossible as well. First of all, Zelda's characters are a special case because there's various incarnations of the heroes. Now, Zelda have the TP look while having moves inspired from OoT and ST, so it's not "one Zelda only". Finally, there's a lot of differences between TP Zelda and SS Zelda. I think that if SS Link is only a palette swap, it's because it's quite close to the green tunic. There's just long sleeves, no chainmail and no hat. As alt costumes tends to have their own colors, I think this kind of costume was unecessary.
Plus, even if the differences are small, a new 3D model is a new 3D model. As Link is rather complex, it's maybe also a question of the 3D model "size" on the disc (but I'm not an expert about 3D modeling, so I'm just theorizing). Imagine if Samus' suits were all actual 3D models.