I propose a monthly subcription that enables premium time for that specified lapse (until the following sub renewal). This would be in replacement of gold purchase to get premium time. For this to make sense it should be significantly cheaper than the equivalent gold purchase for 30 days of premium, given that it has no other use than that.
Gold would still work for pretty much everything else as it currently does. I suspect it could translate to a somewhat larger cash influx for Wargaming, and an opportunity for more players to enjoy the benefits of a premium account, at a more affordable price.