Let's see...
1: Primary light - a security feature if it is dark. Sometimes it may bring some joy.
2: DSMB - a security feature if you dive at sea or if there is boat traffic. One per team.
3: Spool/reel for deco/cave diving is a no brainer. A trilobyte (a cutting device) is needed if there's stuff on the bottom. A camera is a great source of joy.
This summarizes the thing. You will want to see things (lights), you will want to be seen (DSMB, lights), you will want to be able to resurface (spools, knives, ...)