A sim which offers the sort of career experience Codemasters have so often done best, particularly in the original Grid and the most recent F1 games, however more fleshed out.
Codemasters would only need to add GP3 and F2 to their F1 games at the start of their career mode to create a far more complete package, even if the underlying physics aren't the most realistic or whatever.
Hearing that the visual novel-esque interview segments from F1 2010 are to return fills me with mixed feelings, as I doubt in an officially licenced F1 game they are likely to be fully fleshed out.
A heavily career based racing sim meets visual novel or racing sim meets role playing game could be a very interesting experience, albeit one with a likely target demographic of about 7 people.
Another thing I would like to see sim racing games explore more is a variety of approaches to racing car design. In an era where most series are ever increasingly homogenous; looking at how oddballs like the Tyrrell P34, the Nissan GT-R LM Nismo, or the Chaparral 2J could be developed to compete on relatively balanced terms alongside more conventional designs intrigues me greatly.
Say what you like about Gran Turismo, but I can't help but respect what Polyphony have done with the Red Bull X-Cars and the Vision Gran Turismo project in exploring some of these ideas, even if some of the vehicles that have been imagined have been fanciful at best.
The market for realistic simulation is already well catered for, and with sims like RFactor 2 and iRacing in continued development, and Assetto Corsa Competizione looming on the horizon, something with a perhaps more "physics sandbox-y" feel to it could also be in high demand.
Something with a powerful but easy to use track editor/creator, tied to a high quality physics/driving model; would perhaps fill one of the largest gaps in the market. Allowing people to then edit things such as atmospheric pressure and makeup, for example, or the strength of gravity on said tracks, would only make for an even more enjoyable experience.
Overall, there are several clear gaps in the sim racing market, but sim racing is niche and overcrowded as it is, and whether or not these gaps will ever be filled is questionable, but I'd love to see it happen.