An issue many people have with sieges is that nations can call swarms of randoms that have never played CCNET before and hand them a gun to fight in their war. These randoms never stay and overall just hurt the experience of CCNET.
Instead of a rank requirement which requires grinding (something many people don't care about), implement a weekly playtime minimum to participate in the weekend sieges. If you've played at least 4 hours in the past 7 days you may participate in sieges. If else, you may not. People who can't spend four hours a week on CCNET don't have time for sieges anyways. It's an hour counter over the course of seven days, so active siege participants will get their required play time anyways even if they only like sieges.
Pros:
> No grinding.
> No clans who never play invading the server to swing wars they don't care about.
Cons:
> None.