To be honest, I personally would prefer the greatest powers to mostly take up the slots, with one or two wildcards thrown in too. I wouldn't find that boring at all though - these are the greatest powers in history! And they could be portrayed so differently each time, with different leaders and units, etc.
I just feel this is a franchise all about the history of civilization. So the major players of history should be there, compared to, for example, Australia, which is an incredibly young country. It's history has barely started and hasn't made a mark on the world comparable to the greats of history. America on the other hand, even being a relatively modern civ, has had a very large impact on the world and so doesn't really feel out of place in the game to me.
That's just me, I realise many people enjoy having these civs in the game. My bias towards ancient history and relative lack of interest in modern history would be a factor too.