Dynamic Rates
It would probably be beneficial for both makers and takers to be able to define dynamic rates, or more specifically a fee variable (a percentage, like 1.025), that would be multiplied with the last fetched price of the concerned token.

A maker ideally doesn't want to recreate an offer each time the price fluctuates a few percents up or down.
A taker looking at an offer is probably not so much interested in the exchange rate itself, but how much he is paying over current market prices.