A 5-6 player game would experience a lot of problems.The game is perfect as it is.
They could add more NEW Neutrals,More NEW General Cards, Scenarios, 1 NEW Lord(with special abilities,like bribing neutrals controlled by another player) for each faction which can be moved always on your turn, who could ¨heal¨ routed units for a price and buy NEW neutrals from NEW map tiles with NEW cities, all these with the NEW gold resource )
I´d buy 5000 copies of this game too make it worthy for another expansion if this is what holds FFG back from it.(if I´d have the money). They could even sell me a 100$ big box full of chinese air and some tokens.