I buy each slot monthly:
2 Weapon Slots
1 Trinket Slot
^Which in my view is worth it, but the negative is that it only last 30 days. I know other players who hate this as well.
My Suggestion:
Have a life-time price charge us 4k or 5k Ce to get lifetime slots.
Not so sure about the lifetime slots, but I'd definitely like to purchase a longer-term slot. Like a trinket slot that will last 90 days but only costs 400 CE (discounted to encourage bigger spending? idk)