I'll also have to disagree with the idea. As many said above, when you're new, being in the top 50 ranks is a fun idea, but most new players understand they need to practice more before reaching such heights, and won't have all their fun killed over it. They'll just keep playing and feel achieved when they can earn those ranks through effort in training. I also think this may also effect multiplayer experience. Nobody wants to play in a 10 player room where they need to find a beatmap that everyone can access. This would give Osu!'s player-driven community less freedom, and not give enough fun to make up for the loss it would bring. And let's say you take a break from Osu!, and come back with a different level of skill. You'll be stuck with whatever level of difficulty your account is on, and nobody would want to risk making a new one to fix that. And yes, I know you said it can be toggled, but that would bring confusion to new players on why some maps are playable and some aren't. The idea of new players being able to compete with each other on the leaderboard isn't a bad one, but a feature like this wouldn't be worth that luxury.
EDIT: I think the better way to go about new players a chance to compete with each other is just give the ranking list an optional level filter (like filtering country when looking at the list.)