The main impediment is that there is no such thing. By creating the 'ultimate' game you imply your game will please everyone, or at least the majority of people and this is just in the MMO crowd. You bring up some points, however, you never looked at the main component, the combat. What game would you base it on? Everyone has different opinions on what they want the combat to be, how it should play etc. With such a broad range of expectations, you cannot please everyone and so you cannot make the ultimate game just by its very definition.