- puncture kit
- small selection of tools (allen keys and tyre levers)
- saddle bag (or similar to keep the above in)
- pump
- bottle cage
- bottle
- lights
Technically, those are important things, but they're a bit worthy.
I'd go for
1. One of those brass bells that when you ting it resonates for ages.
2. Some particularly ludicrous lighting - maybe those things that paint patterns on your wheels as they go round.
3. Internet enabled valve caps.
4. A display that projects a motivational message on the road in front of you. Comes free with the entire collected quote of Jens Voigt.
5. Novelty cable end-caps
6. Something in the way of mobile phone apps. Probably iphone-only. Maybe one that reminds you to comb your beard when approaching fashion sensitive espresso stops.
7. A stem cap that flashes red when you approach Strava segments.
There you go. Must-have accessories.