This idea was inspired by a limitation that irked me, and the goofy shit I did in Highfleet.
When a submarine is saved to a campaign, it cannot be exited. Any edits to the core submarine file do not apply to that snapshot of it. While it does help prevent save corruption, it also locks out mid-campaign sub editing. What if there was a way around that?
In this idea, a submarine upgrade merchant would allow you to make edits to the submarine. However, unlike the regular editor, there are limitations. You must pay for installations, supply materials for hull modifications and wires for wiring, you cannot change the submarine's class, the submarine is not saved as a new thing but rather just as a campaign save-specific modification, and you cannot change stats such as health, speed, capacity, or rate.
The original is also always kept as a backup in case you forget to add an engine!