Every time I've ever seen a H&K MK 23 I've told myself I'll own one one day. Ultimate grail status would be with the early, chunky LAM, but I know I might as well wish for a rainbow-farting unicorn. I owned a cheap airsoft MK 23 as a kid, if that's any proof.
Outside that, a SITES Spectre M4 fully kitted with the foregrip, top-folding stock and coffin magazine. To me it's the coolest looking of all the traditional SMGs ever produced. I saw one at a gun show years ago and the guy wanted an arm and a leg as expected, but hardly knew a thing about it and it was bare-bones as could be.
Finally, a Mateba autorevolver. Recoil operated, bottom-barreled revolver made in Italy that looks 100% like a space gun? Yes please. I may try to scratch this itch one day with a Chiappa Rhino.