Ever heard of "The Osbourne Effect" ?http://en.wikipedia.org/wiki/Osborne_effect
There aren't a LOT of features you can add to an Arduino and have it still be an Arduino. One possible new feature is support for the "Real Soon Now" ATmega328 (twice the RAM, twice the flash, of the mega158), but that SHOULD just drop into the existing boards' socket (diecimila, NG, original, or even serial.) Were you looking for anything in particular?
I think we can be fairly sure there will be a new Arduino at some point in the future. But that's no reason to hold of purchasing now. You just don't know when the new board will be released, and if you live your life waiting for the next great thing, you miss out on too many current great things. The time to buy is when you want it, not when the new version comes out.
As for speculation of what they could add. Base an Arduino on a mega128 chip to have a lot more IO is my favourite idea, but there's others too.
Still, I wouldn't hold my breath waiting for it and delay buying. It's only a $35 board. If you buy today and a new one 100 times better comes out tomorrow, it's not that major a loss and you can still find plenty of uses for the old one.