What do you think about random lock times for transactions to make instant transactions safe?
If you have a random lock time for special accounts an attacker can't predict any more who will be the forger of the transaction. So an attacker could not attempt to doulbe spend by replacing the transaction with another.
The transaction would be immediately broadcasted to the Network, but have to wait a random time until added to the Blockchain
Example implementation:
Every forger adds an one random number (only 1 or 0) to his forged block on the Blockchain.
A transaction for these specific accounts can only be added to the Blockchain if the last 8 bits of hash of (the random numbers in the last 100 Blocks plus the accounts public key) are zero.
Advantage of this is that you can spend the entire account balance at once.