The idea of rewards to the best players is always a delicate subject, knowing this game is in a big part a pay to win game or, as some players say : with the doubts of cheating, or even with some classes advantaged compared to others etc. Even without this, giving (good) extra rewards to the already best ones could become too much advantageous for only a few. So motivating yes, pretty good for the first ranked yes too, but pretty bad for all others who never manage to be first. Lots of complaints in perspective lol.
But I agree that a monthly thing could be incentive to stay active. Maybe not create fully different rankings all based on the last results (if no reward at the end of each month, it would be meaningless), but adding to the current general rankings some conditions of activity during the last 30 slidding days ? To incite to be more active for those who care about the rankings and allow a bit more easily active new toons to be well ranked.
Like : for the FF ranking, a new column counting the number of participations in FF during the last 30 slidding days may be added, and be taken into account in the final score (and so in the global FF ranking). Good players who used to have super good stats would keep their good stats and it would still lead them to good ranks but they would be less well ranked if they aren't active anymore in FF compared to others. Incentive to stay active for old players who care for the ranking and for new players too cause it could give them a possibility to be better ranked more easily than without this column.
Same with duels : the number of duels initiated during the last 30 days should be taken into account and not only the experience level. Same with crafting (number of products crafted in the last 30 days), construction (number of hours spent), adventures (number of games played). In the towns ranking, I'm not so sure the recent global activity of members isn't already taken into account in the column "member points" ? If not, maybe it should be and with more impact on the ranking.