There will be leaks, but it will take a while. So far, only skins are visible in the in-game preview. Cosmetic items can also be seen in the mode art.
Interestingly, Epic has added new styles to the Skull Trooper. In addition, the game will have an emote that changes one of the characters. In terms of cosmetic items, the studio did a great job.
Mentioned emote that changes skin:
The rest of the cosmetic items, i.e. pickaxes, backpacks, etc. should be available in a moment, but it will likely be shown simply in-game exactly like the emotes: