thinking about gaming (sort of)

So, I saw my cousins on New Year's Day and we played Singstar and Rock Band. And it was fun, but honestly, I think there were only three songs on Rock Band I knew well, and maybe two more that I sort of knew. And of the four versions of Singstar they had, there was maybe one disk's worth of songs that I would be interested in singing. And it got me wondering. Would it be possible for them to create some kind of on-line service where you could create your own custom-made game? Like you select from a list of available songs and they'll make it for you? Because it's not just a matter of there not being enough songs I like per disk, it's that I have pretty esoteric tastes in music so a lot of them are not ever likely to be put onto a singstar game (and my dream of a Broadway Singstar is sure to never come true). Do you think this is plausible? Or would it be cost-prohibitive?