I agree about Sheik and that she deserves to still be on the roster, as for the "representing the franchise" thing, well i also kind of agree, though at this point almost no one but the 3 triforce weilders can do that. I think Ghirahim is more prominent and memorable than those guys too (Vaati, Zant, etc...), but i guess that's up to everyone, so yeah i'm not saying Ghirahim's chances are high, but i don't think he's the least likely either (I mean there's Groose and Epona for that
).
There's also the Trophy theory but i guess that could point out at other Zelda characters.
The only other reason i personally have is that Sakurai seems to be going for more recent appearances for characters now, even Little Mac who has his design based on the 2009 remake, and of course Palutena being based on her Uprising design, i know it doesn't mean much, but i think it can happen.
Another thing is that in the end it's kinda hard to know what Sakurai might choose for characters, i mean in the end if he thinks a character would be good for Smash he'll probably add it, look at Robin, whom he chose for having more unique abilities over Chrom, my point is that i don't think any of this arguments about appearances and importance might hold much weight, especially for a franchise who already has the characters that it NEEDS (3 triforce weilders), if Sakurai considred Ghirahim, wich he probably did considering how much he is representing Skyward Sword, and thought he had potential, then he would have added him, and in the end i think that's what matters. (So in a way even Groose could be added).