Right, I said you "could make music that qualifies as bluegrass." But it sounds like you'd be one of the people who didn't acept it, unless it had the groove, the harmony, the structure of a 'bluegrass song'
Which is fine, and I think that many genres have a lot of growth right in the middle. I mean, look at rock music and how much people have done within very tight constraints (by adjusting sound, playing style, rhythm, etc.). But the people who everyone talks about for years are the bands that were on the edge, trying new stuff that was outside the bounds of "normal" rock music.
I think that's true for most popular music genres, if not all. There's nothing wrong with challenging your band to write really awesome tunes within strict genre limitations.
But there's also nothing wrong with breaking convention if, to your band's ears, it makes better songs. Even if it's not as easily classified, or it challenges listeners. I mean, it would be simple to write new old-timey songs, but would that paradox be accepted by fans of the genre?