1) you must consider the lifetime of the server in a month basis like this: 1st week - C grade, 2nd week - B grade, 3rd week - A grade, 4rth week - S grade.
becouse now it is too fast. rank restriction can be removed.
2) many players when server comes to S-grade fail to enchant to +16 and they leave, so you can think the failure not to return to +0 but instead to a penalty like -3 or if its above +10 to return to +10 or anything else...
3) Must be 2 siege competitions every week. One for clans (as there is) and one for factions. The members of the wining fuction during the week have xp/sp adena/drop increase rate. no change faction must be availiable and no restriction in the participation player numbers. So you can announce the faction winner at the end according to this siege results!
my best wishes!