I do find it rather tedious and money-consuming to divert from my current objective to get more slots for next month, So why not make a DLC to have them forever?
The pack is simple. Two permanent weapon slots, and two permanent trinket slots. For non-Steam users, it would cost around... Eh let's say 2K CE. 1,500 to be safe. For Steam users, $4.99. If you buy it from steam, you have a choice to put on on one of your knights (if you have more than 1).
I'd be perfectly fine with this but as a "subscription" rather than permanent. The slots are supposed to be there to be a somewhat premium service along with Energy: Pay to get additional help.
I'd be perfectly fine if they made a subscription service for however many months you'd prefer. Have the base price be higher than what it currently is monthly but decrease the price up to a certain percentage based on how many months you purchase at once. (Much like how Energy purchases work)