I really liked the AoD "retooling time" concept.
What would happen in reality is that the more units you are expected to deliver per month, the longer time it would take to set up the assembly line = longer retooling time. Especially early in the war before you have experience with large scale assembly line production.
So for HoI4 I would like to see a general practical value for each branch representing the overall knowledge of producing something. This would impact mainly retooling time in such a way that you only can build the really huge assembly lines capable of spitting out thousands of units a month late war when you had lots of experience. Attempting to do this early on should be possible but result in your IC being "locked" retooling ( and thus useless ) for say 6-12 months or so before it's all set up to spit out large amounts of units. This is also a trade off since even the first tank rolling of the production line will be using up to 1 year old technology.
The simple formula is that the more units you expect to build in total, the more worth it it becomes to delay production start a bit to spend more time on perfecting the assembly line.
It would also be cool if this is a player input choice (for example via a slider):
Lets say I'm launching a new production line of a future tank, I may want to move the production start one month ahead to increase production from 200 to 220 units per month, even if I know that it will take 11 months to "break even" before I will have gotten an equal amount of tanks out of the line.
Or if I badly need a heavy tank ASAP I may put the slider maximum in the other direction = I don't care if you have to assemble them manually on by one in a workshop, I need those tanks now!