Assemble and install USB controllers ... I think issues about licensing may come about if you attempt to "sell" the "USB controllers" but I think selling a "USB converted keyboard" is a bit of a different deal.
An easy solution is to simply install a USB hub (sans plastic casing) into the keyboard chassis, with it's own "pass through" USB cable. The keyboard cable and USB cable could be sheathed together as a single unit, splitting apart a few inches before reaching the host PC (allowing a little leeway to connect to crowded I/O panels). No additional electronics signaling (or legal) difficulties are encountered; the keyboard and USB hub are essentially discrete appliances that just happen to share the same chassis.
A similar pass-through approach can be used with 3.5mm audio (headphone, mic) jacks.
Many gaming keyboards marketed today use this approach.
Many also feature gold-plated connectors. Although people tend to think this is a good thing (especially with audio), it really isn't, and the gold-plating is mostly just a marketing gimmick. This is because virtually every USB host or device is tin-plated, most PC-integrated audio ports are also tin-plated. Tin- and gold-plated connectors should never be mixed, since the mismatch promotes a type of rapid corrosion called "fretting" which eventually impedes reliable electrical signaling.
I can think of colored keycaps. You can charge a customization fee ...
+1 more, this is an excellent idea.
Most people (outside GH) think all keyboards are much the same and wonder why anyone would ever need the expensive ones. Perhaps they might pay more for a Microsoft or Logitech keyboard simply because of a vague notion that the brand somehow offers better compatibility and reliability.
When they think of customization at all, they'll think about the cosmetic sorts of things they see on niche-market keyboards. Stuff like backlighting, custom surfaces which match their mousepad/decor, and custom keycaps in different colours. Some would prefer to dispense with the Windows logos (often replacing them with Tux or something neutral like "Command"). Some want different colour schemes on the Function keys, Backspace, Numpad, WASD cluster, or whatever.
A look at higher-end (or at least higher-cost) keyboards shows that eye-candy is big. Even the most elite keyboard enthusiast will be drawn temporarily into considering some inferior keyboard simply because it has attractive backlighting or LEDs.
You'd do better finding old Northgates that are yellowed and whitening them ...
Again, sadly, most people would probably wonder why you'd pay to restore and repair a broken old keyboard when it would cost less to buy a newer model. This is still a great idea, but I think it would only appeal to keyboard veterans and GH-minded people.
A mod that involves keyswitches or controllers could be lucrative, but I suspect it would probably be more of a commission sort of thing than a mainstay.