As of now, all the characters that are being shown on the dojo are starters.
We don't know that definitively at all.
I seem to remember hearing that Sakurai once confirmed in an interview that he would be showing all of the characters before Brawl was released, because he knew that anyone who played online or looked anywhere on the internet would be spoiled within hours anyway. He'd rather people hear it from him with a bunch of fanfare. I don't know where that info came from though, so I guess it could technically be a rumor. Does anyone know?
That doesn't definitively prove that he isn't going to save them for the end, but there's no proof that he
will, either.
The only reason we are sometimes seeing "Warning new challenger approaching" is because they are actually a newcomer. Meaning Pokemon Trainer and Diddy Kong are newcomers so they had the label, while Peach (or Bowser) is a vetran so she didn't have this label.
I agree. Of course, I'm pretty sure everyone knows by now that those things are just for hype. Wasn't this proved definitively a while ago by showing the differences between various languages' versions of that image? Anyway, just because the sign doesn't mark secret characters doesn't prove that the characters aren't secret.
This also explains why we haven't seen snake because he will be unlockable. It also explains why we haven't heard anything about Luigi and Jiggs, because they will most likely be unlockable.
We haven't seen Captain Falcon or Ness yet either. Does that mean they're going to be secret characters this time around? I'm sure Sakurai just likes to keep some veterans under wraps so we don't get to a point where we know every single character update from now on will be a newcomer. Think about it...if he had done all the veterans before starting the newcomers, then we would have known who was removed earlier than he might have intended. Besides, we already have three confirmed Mario characters (four if you count Yoshi) and two confirmed Pokemon characters on the site...maybe Sakurai wants to spread out the franchises. (Note that the franchises that previously only had one character on the site--Kirby and DK--have had the last two character updates. The only loners left are Fox and Ike, and since I doubt that Marth or Roy will be returning, Fire Emblem will probably stay behind for a while yet. One of the next two character updates will be Krystal; I'm calling it right now.)
And as for Snake, he doesn't prove anything. Meta Knight, regardless of his unlockable status, wasn't shown until just now. Snake's lack of appearance could be just as random. Furthermore, Sakurai could easily treat the third-party characters differently from regular secret characters. All third-party characters are secret characters, but that doesn't mean that all secret characters will be treated the same as third-party characters. Besides, Sakurai showed Snake in the
very first trailer. Obviously he can't be that obsessive about keeping secret characters secret.
It's certainly
possible that all the characters so far have been starters, but there are no grounds to call it fact.