For me, it was to always bring a big water bottle (or two smaller ones).
I just got back from a hike on one of the hottest days of the year where I ended up walking around 3 hours and several kilometers after running out of water during the hottest part of the day before I could get to a churchyard with an outdoor tap to fill up on. Not fun!
Extra medical supplies is definitely a big one. I always pack extras of the common ones, i.e. painkillers for headaches, plasters ranging from small to large and antiseptic wipes. Never know when they can come in handy!
I carry a level up in gear as injuries while MT biking are usually more severe
Bleed stop packets Gauze pads Wrapping bandages Soap Hydrogen peroxide Rubber gloves Extra joint…
I’ve helped clean up and stabilize people plenty of times. It’s nice to be ready.