if my idea about the multidimensionality of string theory being a crude model for the expansion of space time in all directions is correct, you may want to actually look towards very high number of dimensions in your string theory - infinite numbers of dimensions, even, as you'd be getting a better approximation of the expanding space-time as the number of dimensions approaches infinity. i'm sure that most logic would have suggested the number of dimensions should be minimized, up to this point. it's some math to play with, anyways.