Ok, here’s one to think about on a rainy Monday.
You have the power to bring back from the past ONE piece of military equipment and put it into production. You can fix defects and add equipment. What do you pick and what do you add???
Example: Tiger II with a Rhinemetal 120mm gun
A Tiger I with a 120mm gun and an improved engine and transmission.
A Panther with the same improvements as the Tiger I.
As for aircraft I’d like to see a Fokker triplane built using the same techniques and materials as modern day jets. I would then equip it with a mini gun in front of the pilot and AA missiles on the wings.
nice!!
As far as armor goes, give an M-18 hellcat an M256 12mm gun and reactive armor. Speed, protection AND firepower, enough to handle those 120mm armed Tigers everyone seems to be so enthralled with! 
Oops, that was supposed to be 120mm gun!
I’d have to say a Jagdtiger, with a new 155mm gun, better transmission and engine, and twin stinger launchers mounted in place of the AA mg, and slightly more ammunition space.
I going to change to scope of the question just a little bit (hope you don’t mind). For me I would have liked to have seen the Avro Arrow reach full production, just to put the myths that surround it to rest. Was it as good as many say, we’ll never know.