the 3939 must remain as it is or cease to be produced. no advancement is necessary as it is already unsurpassable - at least in my humble opinion. i know you like the 5959 too - surely these two pieces are above any advancement.
The 3939 is already sold with a second plain back, in case.....
This second back doesn't come with the 5959 and it may not be anti magnetic enough, I'm sure there are people who use it (I can't believe nobody uses it).
Anti-magnetic watches are practical, but, being able to view the sheer beauty of a Patek movement through the sapphire back is extremely important.
I think a few anti-magnetic watches could be added to the line, but not at the expense of compromising the aesthetic viewing pleasure of extremely special timepieces such as a 3939 nor 5959. Maybe a basic Nautilus or sports watch. Possibly the 5119/5196 since they already have solid backs.