If they're not tangible I don't believe they should be classified as an asset. Maybe an accountant can chime in on that issue.
But I also believe the whole slot system needs a rework anyway. New people can't get near Heathrow which simply just isn't realistic. Airports actually encourage new airlines to use them. They don't turn them away because of slots. They find the slots for new airlines. Well in Canada they do, I guess I better not speak for the rest of the world. Anyway the slot system isn't very good in my opinion though it was an excellent addition to the game, just not the way they are now.
As it is, they should be expensed. They have no intrinsic value. They are not resaleable, and have no place on the balance sheet until they are.
They give a false valuation the the airline. I might have $50m of debt, and $100m of assets, but if they are all unsellable slots, my assets are worth nothing.
In RL, if I was a lender, I might look at that airline and think it was worth the risk because I would get 50c on the $ back if it went bust, I would be wrong.
To look at it in an AWS example. Say I set up an a/l early in a GW from LHR, I buy lots of slots, make a good living, but then make bad a/c choices I will BK and those slots go to the fastest mover. If I could sell some, downscale and restructure then the market has been freed up and my game interest continues. It happens in RL too. Small regional a/ls have rebuilt their a/l on the basis of selling LHR slots.
If the ability, within AWS is not there to enable trading of slots on an open auction basis is not there then they should just be expensed and written off.
It would make sense to allow open trading as it would get away from any secret, under the counter trading or swapping that many suspect goes on within alliances and make the busy airports less closed shops as people merely wait for slot drops.
This is the one aspect of the game that I think is a real error.
(Oh, and I think there should be a ticker, or list, of a/c trades - it would show what the going rate for any model really is, open up opportunities for a/l to offer a/c to potential buyers and close a door to intra-alliance dodgy trades. - Sunlight is the best disinfectant)