Allow guild members to test-plunder each other. No resource transfer. Just damage results and replay. Option 1 Possibly by invitation from base owner only, and limit the number of simultaneous active invites (3?) otherwise an organised guild could prevent outsiders from plundering their members' bases. Option 2 Instead of the base owner specifying which members r invited to plunder him, he could hit a Plunder Me button to notify members that his base needs test plundering. The notice could appear in the chat log. In this case, perhaps limit the test plunders to x times per 24 hrs.
This could also function as a form of dueling between guild members...see who can do more damage to the other members base. To avoid people using it as a farming method, it would be important to have no resources gained from the "test" attack.
As long as it was well structured so as to reduce/prevent this feature being abused as @C.Calamity has already alluded to. no shields given out for a loss and islands become fully available immediately after no profit/loss of resources at the end of 'test plunder' limited use ensuring islands are available to be attacked by other guilds If organised well a great idea.. +1
Definitely on the master list. It's difficult to explain to people why their bases suck. Being able to show them would be a lot more fun. I'd even be ok with paying a gem or two to give someone the opportunity to test raid me.