It could be a category of cards or a minion skill. The missions can return the fun, odd, or augmenting things that have been introduced in the world from various quests, events, or crafters. The items should all be consumable and have time limits.
Some ideas include:

Feet: 15 minute boost to swim seed.
Legs: 15 minute augment to strength.
Shoulders: 15 minute augment to dexterity.
Feet: a spring themed augment that allows you to bounce/leap like you do in Nightmare Tide water.
Head: 20 minute augment to boost notoriety.
Legg: 20 minute leg augment to boost currency gain.

Not all ideas have to be practical some can be fluff.
Cloak: 5 minute augment could leave streaming sparkling trails behind you
Feet: 5 minute augment that leaves flaming foot prints or mud prints on the ground.
Feet: 1 minute augment that produces squeaky shoe sounds when walking.
Weapon: Short term (or permanent a slim chance) augment to add a particle effect.

Super Rare things could come from the 10 hour missions.
Brevonic Battery
Low duration boost potions (but as items to keep the tinkering theme).

The core concepts are fun, time limited, nothing game breaking, tiered possibilities for level and/or rarity. These items could add to the economy if they were tradable as well as long as they didn't flood it.