GT3's F1 cars didn't even make it into GT4, let alone GT5, so there's extremely little chance they'll make it in GT6. I'm going to say a good hard "no."
I wouldn't rule out any new F1 cars though. We could possibly get the F2012 or F138 (2013), although they wouldn't be much different from the F10. We also saw a bunch of classic Lotuses on the Goodwood monument. Of course those cars were there in real life, although notably PD didn't include the 2012 Lotus (which is a different team with no connection to the original Lotus), likely for licensing reasons. But if they could include all the others then there's a chance they could also make the game as driveable models -- but this remains to be seen or even hinted of beyond the representation of this real life monument.
As for IndyCars, if there were any chance of that we would've heard about it by now, just like NASCAR and WRC were announced in GT5 trailers. Although there may be a chance we get a new premium model of a newly designed Polyphony Gran Turismo, but who knows really. Perhaps with the addition of the X2010 in GT5 the PD GT has taken the backseat and is the reason why it was just a standard model in GT5.
Cautious expectations would only really include the F2012 or F138 if anything new at all, but it would be fantastic if those classic Lotuses make the cut.
