I personally think that adding Sebring International Raceway would be so cool! That is like one of the only major racetracks that have yet to be added or planned.
I think Sebring International Raceway (Florida, USA) not being featured in GT is a great pity. It is one of the most famous circuits and hosts big races throughout the year, on it's 6.02km configuration, with the Sebring 12h being the highlight.
Hopefully we will see it in GT7 although I'm unsure if Forza's ALMS rights are still in effect.