On that subject, I'd love to see multiple historic configurations of IRL circuits in a single title. Even if we just included the layouts that've been in the series, there's Suzuka (2003 & 2014), Le Mans (2005, 2009, and the most recent version), the 80s version of Brands Hatch and Monza, plus Fuji (1980s, 2005, etc.).
That's before we get into stuff that could be new to the series, like the far longer Spa-Francorchamps course that used to be around that looks like a triangle - IIRC it was partially a street circuit, in fact!
Then there's also the various changes made to Le Mans in the 20th century - though I wonder what older version of the course would be different enough from the others to warrant its inclusion?