Decent AI is a must for any racing sim, and for me GT's AI is too predictable. Make the drivers more lifelike by providing different driver profiles with different attributes. That would provide enough variety in the driver lineup. GT feels a bit soulless in this aspect because the cars dictate the lap times for the AI, without any perceivable margin for human error.
I don't think GT can be seriously considered a racing sim unless damage is addressed. The damage does not have to be spectacular like in GRiD - it just has to create a similar effect as a car would in real life. Parts don't have to fall off realistically, but when I crash against a wall at 200mph I expect more than a bounce against the wall that lets me merrily continue my course.
GT probably already has enough customization as it is. Custom car paint doesn't add to the racing realism and I think Polyphony already have their hands full trying to balance each car's stock settings to seriously consider aftermarket tuning.
I am not familiar with the unlimited tracks feature, but 20-30 tracks for me is already ample to memorize and become reasonably competitive in. Now if it were "unlimited cars" that'd be something else.
I don't care for downloadable content if it requires me to pay additional. I guess I prefer the mod community approach where it's free. Of course, if Polyphony do offer DLC for free then why not? However, I doubt this would ever happen given their stance on DLC.
One thing I know would take away a lot of the attractiveness of playing GT5 is if they place too much emphasis on DLC while skimping on the retail version. I expect a full game with content at least on par with previous versions of GT, not another "Prologue" with DLC slapped on it.
Having online isn't enough - the network code has to be up to scratch (within reasonable limits of course; lag is lag after all). Take F1 1999-2002 for example: the network code was awful and anything over 100ms lag would have cars disappear and reappear out of nowhere in seemingly random locations. RFactor's network code by comparison is a work of art.
No mandatory installs would be great. I can understand a couple of MBs but not the current trend of 1-4GB (and more) per game on the PS3.
Finally, if GT5 could render the game at 1080p (I'm not referring to the photo mode but the actual gameplay) that would make it "perfect". I think that's a pipe dream though since most titles with complex graphics this generation are barely capable of rendering in native 720p.