I suppose I only gave reasons why Kyurem would make sense to be the cover pokemon of the third game to Black and White.(If you'd like to see them,go to the thread "Black and White version:will there be a "Platinum version"for them?" in the General Disscussion area. You can still read it,but you can't post comments since for some reason,SOMEONE LOCKED IT!!!)
Now on to other reasons as to why it makes sence to have a third game.First of all,according to the myth about Reshiram and Zekrom,they were once the same pokemon.Somehow,I don't think they were some huge yin yang orb floating around.Usually,the third game fills in the gaps of the myth that it goes to.
Secondly,it would make sense for a plot.It could could go something along the lines where you obtain the Light stone and N obtains the Dark stone and you manage to stop him at the top of Dragonspiral Tower.There you two battle,and at the end of it,the two stones form into their initial forms and ditch Dragonspiral Tower.N thinks he knows where to look for them,so he leaves,too.
After a while of talking and myths,Cedric(or some other person.I'm just using him as an example,since he doesn't appear too often in Black and White.) thinks that it best to head to Lacunosa Town.He explains about the myth they have there,so it's settled and you head there,only to find the sky darkend.(If you talk to some of the people there,they'll say the myth of how the "scary" pokemon is coming to get them because they weren't inside their homes before dark.)Taking what you know of the Lacunosa myth and the crater behind it,you head to the crater.Once at the chamber that would be Kyurem's,the two legendaries appear.Reshiram,the seeker of truth,sides with you,and Zekrom,the seeker of ideals,sides with N.You and him have a battle one-on-one.After that,the legendaries merge together with a mysterious figure(Kyurem) to form the cover pokemon.You battle with N again,this time with what you came in with.At the end of it,he heals your pokemon.Now time to face the creature Reshiram and Zekrom once were...
To think that only took me twenty minutes to think up.Only time will tell what the plot might actually be.
The the third reason(this time not as lengthy.)is the fact that it would break the traditions of the pokemon VG world,since every generation has had a third game:
Red/Blue=Yellow
Original Gold and Silver=Crystal
Ruby/Saphire=Emerald
Diamond/Pearl=Platinum
Black/White=????