A dream gun? Well, I do lots of dreaming and not just about guns. I've already owned lots of dreamy guns. A couple I'd like to own again. Starting with those, I'd love to have a good Colt New Service in .45 Colt, 5" or 6" barrel. I had one but it wasn't new. (I also love Police Positive Specials) Likewise, I'd like to have a like-new No. 4 Lee-Enfield. I had one that was practically brand new. That one was made in the late 50's but I can no long make reference to any details. Although it was brand new, practically, it was still strictly issue grade. No Mitchell Mauser.
It is hard to think of anything else that isn't actually available, even in reproduction form. I'm really not very adventurous or particularly progressive, so I'm not ready to have a Savage 99. However, there is one thing I can think of that no one's ever made.
H&R made a run of .45-70 Springfields. I had one of the carbines and I understand they made the long rifles, too, though I've never seen one. I think one with a mid-length barrel (32 inches?) and full stocked (not to the muzzle) would be interesting, even if it wasn't any handier than the carbine. Just different! And oh, yeah, a regular rubber sporting style buttpad.