I realize the bottle is probably already pretty full so I'll break this up in to two groups... stuff I would definitely make room for and then stuff I would add only if there was still adequate room.

Definitely:

MP1 tabs, lighter, matches, additional FA supplies

Maybe:

Signal Mirror, Better Knife, Button Compass, Extra AAA Battery, Cordage or Wire of Some Type (Lots of space saving options on cordage. Could be anything from nylon string to 100# Fireline. I love Fireline but it may be a little more $$ than you want to spend.)