An I2C GPIO expander is a small board with a lot of ways to get the bus wrong. This one is an MCP23017 (16 I/O pins over I2C), with separate pull-up networks, jumper-selectable voltage for 3.3V and 5V buses, and chainable connectors for daisy-chaining.
CopperPilot called out the default I2C address on the silkscreen, poured two ground planes, stitched them with 61 vias, and left ERC and DRC clean. Schematic to fabricatable board, entirely in KiCad.
The same rigor applies whether the board is a connector breakout or something with its own I2C topology to get right.
Try it free at copperpilot.ai.