We need to step back and observe all the races of the game, and go "Well, there are several varieties here. They played this role and all have this in common.". After doing that, we will knock out most races and be left with a handful.
Now, I will say and have said and will continue to say that races like the Margonites, The Forgotten and Mursaat will not be playable (well, at least right off the starting screen

). We will and should call them "Elite Races" for the time being.
Dwarves being playable, along with Tengu and Centaurs is plausible due to our relations with them spanning the three campaigns. We played with, and against them. We seen several types of each, both professions and races inside the races. We seen different organizations inside the races (Deldrimor Dwarves, Summit Dwarves) also in the process.
I know, Dwarves are always being used. But, they are one race of creatures (besides Gnomes...) I won't mind seeing played due to ANET's way of taking them out of the "We just mine, grow beards and grunt" way. I mean come on, even in Forgotten Realms, Dwarves aren't very spellcasting-like creatures. Here, in the world of Tyria, they are.
Centaurs and Tengu follow much of what I said above. Centaurs are often used, but are more often portrayed as just warriors and look like menhorse. Here, we saw a few different races of Centaurs. And Tengu? Who ever heard of a Tengu before they started playing Guildwars? If your not from Asia, read much Asian stories/mythology, odds are you never knew of them before now.
So, right there I would say we have for sure but unannounced: The Dwarves, The Tengu and The Centaurs.
Now, who wants to start figuring out and begin the great debate on who the possibility of the "other" races are? So far we have the five confirmed, with the high possibility of the Three I have listed.