All of these are available on the Bose® Accessories Page
T1 ToneMatch™ audio engine power supply: Provides power for the ToneMatch™ audio engine when used with an L1™ Model I system or computer. Power rating: AC 100–120V~.
T1 ToneMatch™ audio engine microphone stand bracket: Mount your ToneMatch™ audio engine to microphone stands securely with this convenient, custom bracket. Fast, easy setup. Features rotating inserts for a tight fit with a variety of stands.
Long remote cable(s): Not strictly necessary, but you might want to have one or more of these 33 foot cables. I use these on those rare occasions when I need to help from off-stage. But I also use them all the time in 'snakes' with 25 foot microphone cables. It just gives me more flexibility for stage layout.
B1 Cable: If you don't have ready access to 4 wire Speakon cables then might want a spare. Not because they fail, but in case you lose one. What I did was get an extra cable and a Male:Male Speakon adapter so I have a spare, but I can also use the spare as an extension if I need a few extra feet.
Fuses: I've never popped a fuse and you do get a spare with the System. Still, if power is an issue for you, you might want to be prepared.
If you are getting only one System (for now) you might consider getting an extra Remote. If you are getting more than one System then in a pinch you can set things up with a Remote on one System, then move it to another and use it there.
To be clear... I have never had a problem with any of these little things but if you are micro-attentive then you might want to have some of these close at hand.