The EVE and Alganon offline advancment systems are different from my original proposed system in many ways; instead of creating a bank of experience to spend, they offer sequential study avenues that one can queue up to learn. Those are good systems - I'm currenly playing Alganon - but don't go far enough, because much of the game still revolves around how much time one can spend in the game when it comes to character development.
For instance, in both games how much money one has is determined by how much time they can spend in-game, even if gaining game money becomes faster and easier due to offline studies; however, I'm not sure how different it would be in EVE if leveling accompanied ISK (money) generation; in Alganon, the character levels up via in-game activity, but has offline progressing going on 24/7 in addition to that.
In my proposed game, one can set their player to "work" while they are offline, earning an amount of money per hour that is derived from whatever professional skills they have developed; however, they obviously cannot "work" while doing other things while actually in-game, so one of the benefits of actually playing live is being able to earn money in addition to whatever else one is advancing in the background by killing mobs or doing quests.
There will always be some advantage to actually being in-game, but it will not be nearly as significant as most MMOGs today.