Each hero needs different items, but there IS a pattern of sorts. The 6 gear slots, left to right, are as follows:
A canister will always be on the leftmost slot. Its rank and element will match that of your hero (gold energy canister for gold energy hero).
The next 2 slots will be an item the rank of which will equal that of your hero (Hero is gold= gold items). The 2nd slot will typically be an item that requires Throwing Stars and Fuel Cells.
The 3rd item will be a rank higher if they are about to be promoted and gain a new skill (promoting from silver 3 to gold, there will be a gold item) (the latest heroes don’t have a silver item required to achieve silver rank). For platinum heroes, the 3rd slot will always ask for a Core Mk. x in order to promote them to Plat + x.
The last 3 can be items ranging from your hero’s current rank, all the way to green. Items that require Throwing Stars and Fuel Cells won’t show up here.