It's unlikely WWE will add a Royal Rumble every year. I know they have occasionally added a televised Rumble (such as in 2004 when a 15 man Rumble on Smackdown decided who would face the champ at Wrestlemania), but such a thing is unlikely to happen often.
The Royal Rumble is a consistent success for the WWE and I doubt they will mess with it too often. It's appearance once a year is part of its appeal.