Which type of upgrade to my machine mod should i do?

Hello, i'm making a mod that has some machines and i don't know which type off upgrades to implement for then. Here are some features that i've filtered from other similar mods:

- An item that increases something if is in a slot of the machine;

- Tiers for the machine;

- And "both of them": an item that upgrade the tier of the machine.

Please, help me to choose one of them, but if you can suggest other kind off features feel free to suggest them.

P.S.: Sorry about my poor english, i'm brazilian... google translator saves lives...