As I've already given out my Stone-Cold, Lead-Pipe Lock of the Week, I'm afraid my subscribers will have to do with only my Insiders pick on this one.
The answer should be: orcs.
Drow are a heavy favorite, but I can't believe that WotC would put four variations of elves into the core rulebook. What do we know we have so far:
Humans (given, focus = generic)
Half-Elf (described enough to indicate they're in...elf #1 focus = charisma)
Elves (also a given, and elf #2 focus = dexterity)
Eladrin (If I'm spelling that correctly...elf #3 focus = intelligence)
Halflings (we have to get our hobbits on, after all focus = dexterity)
Dwarves (hopefully rebalanced a little bit. <--With their stability bonus that's a funny! focus = con)
Tieflings (the newcomer! focus = dexterity)
XXXXXX (our mystery race)
So what are we missing? We have races that are all about Dexterity (a lot of them, actually...maybe too many...that's why no goblins will be there), Constitution, Intelligence and Charisma.
So what are we missing? Wisdom and Strength. The race that's iconic for Wisdom is the Aasimar (oh, spell check, why do you not have D&D spellings in you?) and Strength.
Is there a more iconic ability score for D&D than Strength? I think not! Therefore, our mystery race will be all about Strength, and what race is more iconic for strength than the orc? Well, okay, both Goliaths and Minotaurs are, but they are also not nearly as popular with D&D players, so Orcs it is!
Note: this isn't a Stone-Cold, Lead-Pipe Lock of the Week, so there aren't any guarantees this is going to actually occur, it's just my best judgment.
--Steve