Alternatively, we can have the reward increase every time we beat off a plunder, potentially getting more reward than the base amount even if we eventually lose (after beating off say 2 or more plunder previously)

You do realize only top 50 CAN attack right? Good luck getting your low level alt to top 50! This isn't impossible per-say, but if your alt is that high up, your main is probably perfectly capable of plundering, which most people prefer (I'm against it on principle but that's a rare case)

It would take a very very dead server.