I have been thinking a bit about the He-111 and how you might go about increasing its defensive capabilities
First off, dump the MG 15 at the outset. They are rifle caliber, had a rate of fire around 1000 rpm, but used 75 round drums. This meant that the gunners spent more time reloading than shooting.
Go with belt fed MG 81, or better yet, the MG 131 and get some heavier hitting power. These changes could have been made in time for the BoB.
Also replace the upper flexible gun mount with a turret, though this would require a significant design change.
Finally, I think going with the all glazed nose was a mistake. While visibility was increased, it left the crew vulnerable to enemy fire. I think they should have kept the original ”stepped” cockpit, which would have given the crew more protection and or even armored.
ric350