Next step; using Ethernet
In a
different thread, I mentioned that one could use raw Ethernet frames (
layer 1 frames) to transmit RS-232 data between endpoints if using a small FPGA (with a sufficient SERDES), making it routable within a single local area network segment using standard switches. You could even wrap the data within UDP/IP frames with configurable source and destination IP addresses and UDP port numbers, making the traffic ordinary UDP/IP.
The point there was, wrapping the datagrams in an Ethernet frame does not actually cost that much; you definitely don't need a full IP stack at all. Only the 32-bit checksum (0x82608EDB, x³²+x²⁶+x²³+x²²+x¹⁶+x¹²+x¹¹+x¹⁰+x⁸+x⁷+x⁵+x⁴+x²+x+1) at the end (but before the interpacket gap) needs computation.
To drive the SFP module, one could use e.g. Microchip LAN9250 Ethernet controller (7€ to 9€ in singles at Mouser), communicating with it via
8 or 16-bit data bus and SPI from basically any microcontroller.
I do realize OP asked explicitly about
other than Ethernet, but my point is that you can easily offload or use Ethernet on the wire, without specifically targeting Ethernet in your MCU. If you look at the Microchip
EVB-LAN9250 SFP interface (FTLF1217P2 on page 6), you only need four pull-up resistors (2×82Ω + 2×49.9Ω), two pull-down resistors (2×130Ω), four 100nF capacitors in series with the four data lines, plus some 3.3V supply filtering and mode strap resistors for the SFP module that will depend on the exact SFP module used. In particular, one might use a separate slave microcontroller between the main microcontroller/device and the LAN9250 chip, converting the data from/to the custom format or data stream, to Ethernet packets; a kind of a custom-protocol to Ethernet + SFP driver module. Replacing that module you can switch to a different interconnect, without chaning the main data stream format at all.
TL;DR: Having your main device know nothing about Ethernet or the interconnect used, it would only cost about 20€ in parts in singles from Mouser to implement an Ethernet-over-SFP interconnect using 100Base-FX (like the Broadcom AFBR-57E6APZ-HT SFP module OP linked to), if you use a translating/buffering microcontroller and e.g. LAN9250 industrial 100Base-FX ethernet controller. To switch to a different interconnect fabric, you just replace the microcontroller + LAN9250 + SFP module, with no changes to the original device, maximizing upgrade path and long-term maintenance opportunities.