It'll be modern, let's face it. I don't mind that much, I do actually enjoy a big modern battle once in a while. I just hope they have more armies than the USA, China and 'MEC' from the start again - obviously the Brits would be nice, or the Germans, but I'd settle for the 'EU' again, like in BF2: Modern Combat.
In terms of player numbers, it needs to be at least 64. And I have a feeling this will indeed be a PC exclusive, as BFBC2 seems geared towards the console market, freeing the PC team up to produce a more taxing (but inherently better) BF game for PCs. So here's hoping.
Oh, and I still think that PDF from 2007 was bullshit. So much stuff on there was what everyone was predicting and DICE themselves blew it out of the water when they said 'we don't comment on rumours and speculation' - practically stating that it was just speculation.