I do know that this is the last SSB that Sakurai is going to work on. However, the characters we know of now (plus the really obvious ones [Captain Falcon, Jigglypuff, Ganondorf etc.] and all the ATs and trophies; not to mention all the decloning they're probably doing for the likes of Ganon and possibly Falco) are already a fairly large upgrade from melee and a rather good representation of Nintendo's gaming legends.
Correct. However, there are still quite a few popular Nintendo characters who have not yet made it into the SSB franchise as playable characters. As this is the last SSB game Sakurai will work on, I'd imagine he'd try to please the fans of these characters by including them (or at the very least, some of them), since it's not like they'll have a potential sequel to look forward to. Many of these characters have huge fanbases and honestly, I think it'd be no less than foolish on Sakurai's part to not include a single one of them just because he's added so much other content already. Characters are the focus of the game, after all; while assist trophies, pokeballs, stages, game modes, and so forth contribute to making the final product an extremely pleasing one, the characters are going to be the real draw. Special attention needs to be paid to the roster in a game like this.
Krystal, K. Rool, Ridley, Midna, Geno, and etc. are all well-liked characters, but many of them are either secondary to the actual heros of their franchises. And though Metroid, Star Fox, and Donkey Kong are gaming icons, they aren't so well known amongst the general populace that people would see Ridley, Krystal, and K. Rool and say "I know them!"
Once again, you're correct. At the same time, this isn't the original Super Smash Bros. game we're talking about. What do I mean? Well, that game was pretty bare-bones in terms of characters. A pretty small roster featuring only the main characters of Nintendo's biggest franchises (Mario and Pokemon excluded). With Melee came the secondary-but-still-popular characters and those from smaller, forgotten, and unknown franchises. Many of these characters (Marth and Roy, for example) were virtually unheard of - at least, internationally - prior to Melee.
With Brawl, we're seeing a similar expansion in roster. We're getting additional secondary, unknown, and forgotten characters that were unable to make it into Melee. Once again, much of the roster consists of characters that most people won't see and go, "OMG, it's SO-AND-SO!" I'd have trouble believing that most people recognized Pit, Ike, or even Zero Suit Samus. However, the recognition that these characters did have, among other factors, was considered sufficient enough to include them as playable characters. I really see no reason why Sakurai's mindset would suddenly change when it comes to the remaining popular-but-not-quite-iconic characters such as Krystal and K. Rool.
I know that popularity and recognition aren't the only criteria for getting in Brawl (Dedede is fairly well known in Japan, but I doubt he nearly as famous overseas), but I still retain my doubts about all these secondary characters making it in. It'll be a pleasant surprise if they do get in (though not one that I would find actually shocking), but I'm almost certain there will be hundreds on these boards a few weeks later screaming "Die Sakurai! How dare you not include Krystal!"
I have no doubt in my mind that Krystal will be included. I think you're really underestimating exactly how popular and significant she is if you think she won't be in the game as a playable character, especially given the inclusion of some of the characters I mentioned above.