For me its a Semiautomatic 357 magnum carbine with a 16 inch barrel.
Ohh yeah. As soon as I saw the thread title this was the first thing I thought of. I'm imagining something with a tube mag and bottom loading gate like a scaled-down Mossberg 930 shotgun. It would come with ghost ring sights, and some kind of manually adjustable gas system for shooting light loads. It would be great if you could shoot .38 special in it but I have no idea if this is possible in a semiauto.
My other rifle wish is an M14 .22LR trainer. I've seen some pretty cool 10/22s modded to look M14-ish but none of them are quite right.
Since I'm dreaming already, why not an affordable M14 produced by a highly modular industry like that which supports the AR15.