Don't you think that more advanced diplomacy and more politics are pretty much the same thing? You can't really have one without the other. I think these two would make the game better than any of the other options. With better politics, empires would take a more active role in cooperating to counter the largest most aggressive civs.
I see so far that tactical battles has the highest response. I thought I would miss it but I find now that I am playing GC2, I like the current tactical battle setup.