I've been contemplating how something like this could work for awhile, as well. The problem is how to implement it that the whole key is exempt from the Tutorial. After all, I've got every ++ character currently unlocked and I still get the "Don't forget to read the FAQ" message every time I make a new character, after all. I thought it might be tricky to set up so once = whole account. As for this, I have a few ideas:
1) The tutorial IS every time, but you benefit from it. Such as having an AI partner who chops through groups easily to help powelevel to 5-10, or perhaps a slew of items given to you. Something to make the tutorial actually worth playing every time, you know? I've seen multiple MMOs with mandatory tutorial areas, but the rewards for doing everything there compared to running straight to the end are always worth it.
For example, Nexon might be a horrible company, but the tutorial island in MapleStory, if it's still the same as all those years ago, has some nice rewards. Low level quests to hit level 10 easily, a new weapon, new headgear, and a chair (recover 50 HP every 10 seconds as opposed to 10) as well as potions. If you started off on Victoria Island instead of Maple Island, you'd be hard pressed to even survive.
2) A verb to enable/disable the tutorial can be given to everyone who clears it once. I know that verbs like mod powers are account-wide, even affecting new characters, so this way you could choose to do it when you're bored but you can usually disable it. The problem here is how to aqquire said verb. The only one I know we can actively earl is vaccine, which is limited to one character. Is making an account-wide toggle even possible?
3) Two words. "Slot one." Though not everyone does, most players start with slot one as their first character. Thus we could make it so the tutorial only applies to slot one, somehow. They'll need to do it again if they delete and restart slot one, sure, but chances are, that's a few ++ classes away.
I know I have more ideas but I have this bad habit of suggesting too much at once.