I added this idea to Uservoice. If you like the idea, you can vote for its implementation on Uservoice at
http://warlight.uservoice.com/forums/77051-warlight-features/suggestions/1521999-ladder-seasons-
Copy/pasted summary:
This would cause the ladder to run for a determined period of time and then reset. The time period would have to be determined, but 3 or 4 months seems reasonable. At the conclusion of the 'season' final rankings would be permanently posted to a player's profile if they finished in the top (insert arbitrary number, such as 10, here). This would allow newer players to improve during the season and not have their early learning curve losses haunt them forever. The conclusion of a season could also see a massive tournament with the rankings used as seeds to determine a winner for that season.