The TrinketItem class only extends the Item class, which makes it difficult to add special things such as weapons into slots.

So i think making it to an interface like Curios API is better.

Trinket is an Interface, which TrinketItem implements.

fzzy is correct, the interface you're looking for is Trinket. TrinketItem is just a class that extends item and implements Trinket with auto registration, for convenience sake.