Hmmm. Bringing makeup and brushes to a convention depends on my cosplay list. My last convention I didn't practice my makeup beforehand, so I brought a bunch of pigment samples I didn't even use.
So that's my advice. Practice your makeup looks before your convention, and you'll know what to bring. As for things to carry while wandering the con: powder, blotting papers, and my lip product(s) of choice. And eye drops when I wear my contacts.
And here's my general makeup bag list: two face primers, eye primer, eye shadow, pencil and gel liner, brow powder, foundation, blush, mascara, lashes and lash glue, lip products (liner, lipstick, and/or gloss), and any other items I need per cosplay. I also bring my bag of makeup brushes and lash curler. It is an awful lot of stuff...