An mmorpg on pc/mac where the skill system was always completely accessible, except that skills had enormous learning curve. For example, learning to jump would be easy, and would only require hitting a given key to perform the action. But learning to fly would initially be practically impossible, in that it would require outstanding keystroke consistency and complexity; however, the more one flies, the easier the task becomes, quite literally, via means of gradually less difficult commands. This would work the same way for every skill in the game, where players would literally have to become more and more skilled themselves with a keyboard to advance in the game. Changes would take place gradually, and be dynamically interconnected (i.e. skills requiring comparable actions could compliment one another); there would be no levels so-to-speak, though progression could be documented by degrees of mastery in each skill.
Remember, each skill would become easier, but start out very challenging to perform, and would have to be learned in-game (i.e. no instruction manual), via NPCs and other means. The control system would be very intricate, allowing for combat to be extremely dynamic.
Outside of all this, the world should be of a futuristic fantasy sort, though not necessarily to an extremely. Engineering should also have an extensive system construction.