An inline having a slower twist with the right type of rifling that's conducive for shooting patched round balls wouldn't be such a bad idea. It would probably also need to have the option to use percussion caps. And a lot of folks really want to shoot the big bores, so they might even go for an inline if one were made in .58.
Those TC .58 big bores are hard to find and folks can't find enough of them.
Even a .54 wouldn't be so bad.