Personally, I think that it'd be ideal to represent more of Final Fantasy as a whole. Obviously the main focus is characters, but there's only so many times we can say that Black Mage would be a very good and fun addition. My thoughts are on the other stuff.
As of now, Final Fantasy VII is considered its own series, which to an extent? I actually understand the mentality. I mean, there's a lot of media under the FFVII label, and if the next character we see is someone like Tifa or perhaps even Vincent, I'd not be opposed in the slightest. However, I'd say there's a lot of room in Smash for more Final Fantasy content overall. Maybe introduce a couple of stages, some Assist Trophies, that sort of thing? I mean, seeing locations like the Chaos Shrine to represent classic FF in here, as well as perhaps some newer places too, that sounds fun! And I'd adore seeing a Magitek mecha as perhaps an Assist Trophy.
That aside, there's a lot of Final Fantasy to represent, and I think focusing on FFVII is actually a good move from their part. I mean, what if you're a FFV fan and the next thing you know, the only representation your favourite game gets is one single song (Battle on the Big Bridge most likely) and Bartz can't even get in as a Spirit because the other ones are taken up with the task of representing the behemoth that is Final Fantasy? Consider, for a moment, that in this circumstance, it's possible that even characters like Aerith wouldn't have gotten a Spirit as of now purely because of how many characters they need to represent somehow. It's not an ideal situation by any means, but FFVII is probably the most popular of the bunch, plus it gives them space to branch outward from an established "series" as required.
Tl;dr, I'd love to see Smash represent Final Fantasy as a whole, but I feel that Final Fantasy might be a bit too big to really give it justice.