Seems like a good hint, but... who could they add for Metroid considering Ridley is already hinted at in that same trailer as a boss character?
As for Zelda, Ganondorf has yet to surface (he's never been officially revealed in prior games, either), but I suspect he could be replaced with Demise. Impa might be fun too.
Anyway...
Robin is looking to be a very interesting type of character. It seems his abilities are based on his Tomes and the weapon he's wielding at any given time. According to a recent picture of the day, Robin pulls out the Levin sword whenever he does a Smash attack, and can even perform Smash attacks (Meteor attacks?) in the air. But like his tomes, the Levin sword will wear break and it will take some time to regenerate.
Speaking of his tomes, rather than him having four different tomes for each spell, it looks like all his tomes can be charged similar to Samus's, Lucario's, Marth's and Ike's neutral B. At max charge, the tomes will fire off their most powerful spell, but it looks like one of the tomes (the dark tome) is actually a throw and not a special attack. Another thing to look at - much like the sword Robin wields - his the tome he's currently equipped with changes his basic combo attacks. If he's equipped with a Wind tome, he finishes off his basic combo with a wind spell, and with a thunder tome, a thunder spell. So much like his function in Fire Emblem: Awakening, Robin's a very flexible character. And as mentioned in the character trailer, he can only use his tomes a certain amount of times before it expires, this pretty much means that he won't be able to use certain special attacks or charged attacks for a while. Robin kinda reminds me of a gunslinger character who needs to reload after pulling off a number of attacks, and I'm guessing this was sort of the idea for Robin's gameplay.
Overall he looks like he might be a tricky mage type character, but one that might be a tad bit more effective than previous mage types like Zelda. I wonder if he, Zelda and Palutena will bear some similarities with each other. Could Robin be the light-weight mage compared to Zelda and Palutena's middle and heavy-weight gameplay?
EDIT: Oh I should probably mention my predictions for Robin's special attack placement.
Neutral: Thunder
Side: Fire
Up: Wind (note that Robin gains height when he casts a wind spell)
Down: Nosferatu (Fun Fact: Robin can't actually cast dark magic while he's a Tactician or Grandmaster, in Fire Emblem Awakening).
