Smart Dynamic PoW concept

An algorithm may be capable of serving as a multiplier to the already existing dynamic PoW solution. It can be capable of dynamically, and exponentially, increase the PoW necessary for future transactions of a wallet, and take into consideration a multitude of factors. These factors can include amount of coins being exchanged, total wallet balance and time between transactions. Each of these factors can also have a dynamic weight, increasing or decreasing with overall network traffic. As the Nano coins remain still in a wallet for longer, the algorithm slowly fades it's increments of PoW.

Maximize the costs of perfoming a spam attack while the network is under heavy load

By factoring in amount of coins per transaction, total balance and the time between transactions, a spammer will have to acquire a larger amount of coins and spread them in more wallets, in order to minimize his average PoW multiplier.
As the network grows in value, acquiring extra coins becomes more expensive.

A legitimate wallet which transacts more often will suffer a heavier penalty during heavy load times. Having a larger balance will mitigate this.
Can potentially harm network efficiency, as nodes might have to verifiy if wallets fully performed their PoW.

See this thread for an interesting idea along these lines which is currently being discussed:

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Thank you, I didn't know of this reddit thread. I left my take on the subject there.
Perhaps a transaction prioritization algorithm may not be the best approach to tackle the matter. A prioritization algorithm will not reduce the amount of transactions a spammer can get through -- it will simply delude the attack over time.
Work increments, however, will harm the quantity of transactions a spammer can get through. However, it does come with downsides, as pointed in the OP.
Anti-spam protection in the Nano network is a very fascinating subject.