Dynamo lights FTW.
Non-rim brakes. I probably would go for hub brakes but they would be less versatile (I’d be more worried about overheating on long descents if I used the commuter for touring) and, more to the point, are far harder to find in this country.
A fixed gear will do the same job, but I kept breaking things (cranks, BB axles, a stem and a frame) on mine to the degree that the KISS principle broke down
Mudguards (all year round, take the road bike if it’s nice weather and I don’t have kit to take in)
Rack (I always wonder why more people don’t run racks/panniers – so much nicer having the weight on the bike than on you)
Warm clothes and a bike with ice spikers fitted ready for the dodgy ice days in winter.