One of my uncles used a .45 cal inline for a few years. He wanted the "flatter" trajectory offered.
Didn't help him as much as he thought it would... most PA deer are taken under 100 yards, especially where his stand was set up, typically under 60.
Not to mention that at the time, most loads for the 45 needed some distance to drop back into the reliable expansion envelope (too close, they'd fragment before penetrating deep enough.)
He went back to 50 cal for reliability and compatibility (with everyone else in the group) purposes and hasn't looked back.
That said, I'd be interested in a small caliber inline for small game. If someone wants to make one anywhere from 25-36 caliber, I'd give it a whirl.
Especially if they could get a powder company or two to crank out some pellets for it.
Now if Harvester or MMP would also jump in and make some sabots, I'd imagine things could get interesting quick with a 30cal/36Sabot or a .270/32cal combo, given enough barrel length...