There are some I would love to see. Maybe not necesarily in a sim, just a videogame out of it, like:
-Late '90s and early '00s GT and LMP
-The 1995 to 2000 British Rally Championship, with its F3, F2 and Kit Cars
-Something which covers mostly all Group A cars (in circuit racing guisse)
-1990s BTCC, or any series with Supertouring cars
WRC or historic rally. Because it hasn't been in actual hardcore, modern sim
Also, police chases. Not a racing series, but also interesting concept, found only in arcade games. Games for big part is doing scary things in a consequence free environment. Being a getaway driver in an open city is definitely an interesting idea for a sim
Any era of classic F1, really. Or how about a game specific to an era, where you'd have the best cars, classes, and tracks from 1985, for example, liveries, layouts etc. reproduced as faithfully as possible. I just like the idea of historic tracks and vehicles which fit the environs.
The only thing I ask is to have the time to play every championship I have on Race 07...
WTCC (2005 --> 2013 + 1987), F3000, ETCC, STCC, TTA, DTM 2010s-2000s-90s-late80s, BTCC, 90s Touring Cars, Italian Superstars V8, Australian Supercars, Radical, FIA GT, FIA GT2, WTCC Extreme, Trucks, Formula Master, Formula Bmw, Formula Ford, Camaro Cup, Classic Cars of all kinds, LMP Series, Formula Raceroom, Mini Challenge.