There is the issue of some folks having impenetrable defenses. I suppose there isn't an easy solution to this, as it would only screw the weaker defenses. I might have a small solution that would keep the risk/reward ratio somewhat in balance.
During a bloodmoon, the risk of venturing outside your base is absolute, while there is no reward, so not many do. Raid mechanics could be altered a bit during this short time. It would not qualify as easy mode raiding, since the risk could easily be equal to the reward. So how would this not screw the weaker defenses even more? Well, for instance, higher authority towns and larger pclaims could become more vulnerable.