Tradeable Mob Levels
Make mob levels tradeable, in a form of a ticket, of course adding up its not possible. For example:
I got level 15 of demons and buy a mob level 23 of demons, when i eat the scroll my mob level gets replaced, and now its 23.
After a couple of months i buy a level 59 of demons, (im still level 23), when i eat the scroll my new mob level will be 59.
It will only change the current mob level by replacing it and you cant combine them like rep on a crafting vessel.
I can see this being applied like, your main is ss pd for over a year now, you got your nice level 45 of demons but suddenly you wanna switch to mage and hunt frost... well you can trade your level of demons and go seek in the market somebody selling a similar frost level.
The price of the tickets will be determined by the players, who knows how much a level 100 of demons will be worth.
What if you could make mob lvls account wise but one char at a time. You can purchase at city hall a Mob Level Ticket that will remove your mob level of the current char and convert it into a ticket (account bounded) then you can mail that ticket to the other character in the account and "consume" it to gain that mob level.