In relation to the names mentioned a few posts back, I was actually thinking about them tonight and by process of elimination, I actually now think the third game name will be Opal.
I think it just relates to the G/S/C pattern in relation to D/P/Pt, which are the game names with opposing names, in other words, it should match up like Gold/Silver/Platinum and Diamond/Pearl/Crystal. Then I look at R/B/Y and R/S/E, which actually match up as per their previous two games. Because of these two patterns, B/W is the 5th, and the odd one out. (Also notice how the matching names pattern were set at the same time). This means Black and White, both colours, should best match up with something that relates less to the other generations. The two it followed - with "neutral" colours, like Diamond/Pearl/Gold/Silver, etc, being...idk lightish, not really on the colour spectrum (Uh, unsure how to describe it), and the fact that these are Black and White, being neutral colours, too... suggests that the third game won't follow the pattern of matching up colours with colours, or gems with gems and instead a colour that contrasts the two, well, combines them. Opal is sort of like a whiteish colour, which can also be seen to be sort of misty, together that could be the combination of Black and White. Due to this, I believe there will be a grey legendary within B/W that people will be all like "omg new game is gray/grey", when they'll overlook the fact that in B/W, the title legendaries were contrasting, not complimentary colours (well, I think that's what I mean at least), but yeah, logically the third game will be named Opal.
Then again, this is just my theory, so yeah.