You can pull out home equity in a cash out refinance. Borrow at 2.5%, then roll that into index funds averaging 8%, and turbo-charge your retirement goals.
I've tried to diversify to more international mutual funds, but they also tend to have higher fees.
I'm not sure they are really diversification though. The major US companies have a lot of international exposure already so you don't gain much diversification. And of course they don't really protect you from a collapse of the US government (odds are either they are hit as well, or you can't prove you own them anymore)