In each generation there is what's referred to as a 'third game'. First they start with a pair of games, e.g. Black/White, Diamond/Pearl.
Then they essentially 'remake' the game and add a bunch more features they didn't pack into it first time. Platinum is the same game as Diamond/Pearl with a few extras like the Distortion World, Battle Tower and so on.
Since generation III there have also been remakes of previous generations. We had FireRed/LeafGreen, then HeartGold/SoulSilver. Adding another 'third game' to those would be pushing it too far.
The third games that have come out so far are:
- Yellow (gen I)
- Crystal (gen II)
- Emerald (gen III)
- Platinum (gen IV)
No word on gen V yet but I'm guessing it will be called Pokemon Rainbow! Or maybe Pokemon Grey, but that sounds pretty dull.