I'm sorry, what's the problem with changing the limit in IsStandard() back to 80 bytes? I have yet to see any argument for why 40 is better than 80, and using the latter value would benefit everyone involved immediately.
For relay, probably no immediate problem.For mining, it encourages spam.
For long-term, it's unnecessary.
Edit: Except that relaying transactions which will never be mined is currently exploitable. So either that would need to be fixed first, or the relay needs to be more intelligent and only relay Counterparty transactions the same way miners would mine them.
If the issue is spam control, then wouldn't that be negated a variable fee based upon based upon the level of risk that spam may be contained within the transaction? Such a variable fee in this case is non-linear to the amount of 'risky' data attached. It would remove the arbitrary nature of 40 or 80 bytes.
It is difficult to predict what is necessary or unnecessary in the future. What if I wish to publish into the ledger the the daily fixings of the history of a 500 million dollar notional interest rate swap. Perhaps at that juncture in the future we would decide it was inappropriate to be stored in the blockchain. However, if it was appropriate I would certainly be willing to pay a larger premium to place it into the ledger.
