The way it is now, free-users cannot play premium maps or gametypes. Instead, why not provide 1 permanent free-server per-gametype (per-region), which can be fully accessed by all players? On the server, all the premium-maps are playable. That way, if the gametype is worth paying for, the 1 server will always be full, and therefore, will be insufficient for free-players (which can be exacerbated by prioritizing player-slots to paying players over unpaying). At that stage, as their appetites have been whetted, they will purchase a premium account. And if the gametype isn't worth paying for, it will at least attract more players, who will generate more ad-revenue or potentially purchase a premium account for other reasons.

The free/premium/pro system has not been exceptionally successful, but maybe it could be with the right modifications?