We've talked about a stats menu before but this is something I don't think has come up.
Weapon Usage.
I'm thinking % of playtime using a weapon, maybe also time using that weapon in the previous run, as well as damage with that weapon - both previous run and lifetime.
May open up a chance for cosmetics or something - like what I suggested for the parkour challenge before, with the weapon mastery levels of skins. Color palettes for weapons, new weapon trails, etc.
Maybe Bronze at a hundred thousand damage, Silver at 250k, Gold at 500k, Glimmer at a million damage, Awakening Fire at 2.5m, Blood at 5m, and Bola (the rewind cutscene effect) at 10m damage dealt with the weapon.