For those wanting RS-232 expansion card, I’m placing my entry in the hat.
This expansion ard is based on a classic MAX232 transceiver for RS-232 with MCP2221 doing the UART to USB duties. The final RS-232 signal ends up being around ±8V which is pretty much standard for such setup and well within specification (±3-15V).
Card is of non-isolated so all ground loop warnings apply. However, considering vast majority of existing USB to RS-232 cables are non-isolated too, this is nothing new or unexpected. Essentially, if a standard converter works for you, this expansion card will work too.
Card connector is a shrouded 3-pin JST XH header which also allows you to use standard 0.1" jumper cables for testing. For a more permanent setup, a cable with JST XH connector is of course preferred (many such precrimped cables are available on eBay/Amazon).
While I do offer a hand-assembled devices for purchase, the whole project is also available on GitHub so you can roll your own.
A bit more details can be found in my blog post.
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This looks great!
Any reason why you forewent exposing any of the other RS232 signals?
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Mostly to keep it friendly for hand soldering.
If I wanted to expose the additional signals, I would have needed to switch to another UART converter with a higher leg count. Since I wanted to skip dealing with an external crystal, that would leave me with FT232RL as a most likely candidate and I really try to avoid that one.
Further more, I would need a bigger RS232 transceiver handling all those signals (with of course more pins; probably SSOP) and larger size caps. Combine that with bigger connector and everything becomes really crowded. Not a deal breaker, but definitely more annoying to solder.
Also, due to a connector size limitations (if I wanted to keep it 0.1" friendly), I could fit 5 pins at most. Taking the mandatory GND, RXD, and TXD, that leaves enough pins for either DSR/DTR or RTS/CTS pair - not both. And it seems there is no clear standard on which pair is more used for flow control. Some devices like DTR/DSR but other devices like RTS/CTS better.
After going over all devices I own that have a RS-232 port - mostly routers and some industrial stuff, I actually found that not one requires the hardware flow control to work. So I opted to keep it simple and stick to just data. Yes, I know they do exist but it seems not to be as common these days as it was back 20 years ago.
I might create a FT232RL based card in the future but at this point it seemed to me those extra pins would just stay unused. Unless I have a strong case for adding extra pins, I prefer to keep it as simple as it gets. Type-C connector is hard enough to deal with. 
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This right here is what I love about the community, actual polite discord about design. Someone asks a question and gets an actual answer back not just a quip.
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