I think after Episode III Alderaan will not be that suitable for a Republic Army. Alderaan is the primary planet in starting the Rebel Alliance, which is technically against the Clone Army since by that time they have turned into the Empire. I know Kamino isn't the best choice, but I think for the purposes of recognisability to gamers it makes sense.
As for Geonosis, it is the homeworld of the Confederacy, even if it was taken. That's where the treaty was signed by the Commerce Guilds and where the armies of the Separatists gathered. Episode III might present a better homeworld but eventually (since the Separatists aren't around for the Civil War) that will get taken by the Republic/Empire too. Which planet do you intend to make the Rebel's homeworld? Because all of their bases get taken by the Empire. Again it makes sense from a gaming perspective.