What can I get out of fishing?

I spent quite a while in Team Rumble just fishing in essentially any body of water that was in the circle. The majority of my items were Small Fry, Rusty Cans, and small amounts of building materials. In the trailer for Chapter 2, Season 1, they showcase the 4 initial characters fishing up to legendary loot from the waters, yet I haven't even seen a single gun as a result of fishing 100+ times in the same match.

What can I get out of fishing? Is any item that can drop from a chest/on the ground fair game?

You can get materials, Rusty Cans, Small Fries (Common), Floppers (Uncommon), Slurpfish (Rare), and the Mythic Goldfish (Mythic) from fishing.

You can read about the effects of each item from Fortnite's Fish Wikia Page