I'd like to power a chip that draws about 40mA at 3v3 and uses I2C. There should be just 2 wires
I2C has two signal wires and ground: SCL, SDA, and GND. I2C does not work without a common ground reference.
BTW: I looked into 1-wire driver chips, but even with 'strong pull-up' there is still the all-zeroes packet to consider with about 50% duty and other issues. The 28E18 can deliver 10mA to sensors for example.
So? 1-wire only needs one signal wire for bidirectional signaling, so that leaves you the "second" wire to supply current to the chip. Granted, the bandwidth is quite low, 16.3 kbit/s, compared to I2C (400 kbit/s typically). So, then you'd have VCC, 1WIRE, and GND. At the chip end, you have a MAXIM DS28E18, bridging the 1WIRE to I2C, so between the chip and the DS28E18, you have four wires (SCL, SDA, GND, VCC). But between your host microcontroller and the DS28E18, you only need VCC, 1WIRE, and GND.