A 357/38 semi-auto tube feed carbine/rifle.
I've been looking and there just is no animal out there that fills this slot. What a fun shooter it would be, and ammo is sold everywhere. I mean with the crazy's on one hand wanting to do away with all detachable magazines I think someone like Savage, Mossberg, or maybe even Ruger could come up with a simple design that incorporates a blow back design that would be similar to the old Savage Model 87A with a tubular magazine. They would just need to beef up the action and barrel a bit. Being a rimmed case it would fit in perfect with a tubular magazine and mostly flat points are utilized in revolver boolits also. Just need to offer one in rifle and one in carbine lengths. I know I'd be pre-ordering one for sure.
Cast in 158 grain to even lighter loads would be the perfect cast boolit platform for launching these cast loads at paper or even small game. I would think that most casters have molds that are in 38 or 357 design.