I believe that most improvements in GT5 will trickle down from having realistic damage. When I say damage, I'm not only talking of the cars, but the track as well. Driving into a tyre wall should not only mess up my car, I want to see those tyres flying. A guard rail would bend or break if hit at certain speed, etc. As for the cars, I have no tolerance for simple damage à la Burnout. Damage skin 1, 2, 3, 4 does nothing for me. I want damage physics!
Some may say that is way too many calculations for any console to do, but think of the 7 core processors! 1 processor could render the car in full 3D at all times on the track, another takes on the AI, another one displays and records track and track changes and so on.... It's possible and I think we should expect nothing less.
With damage, better AI is necessary: holding a line at all cost would be detrimental to all cars and the very fun factor of the game. This would probably enhance the AI on all levels of play. By that, I mean that not only will the difficulty level be dictated by the size of the engine as it is now, but easier races will have more "courteous" drivers who are more prone to mistakes and giving up the line and harder event will see tough as nails drivers who won't give you an inch (without gratuitously ramming you when you brake too soon).
Also, concerning the career mode: I WANT A TRUE CAREER MODE! I'd love to have sponsors fight for you, external corporate decision out of your control affecting you team, the expanding and bettering of your entire team (from the pits to management) and some history... Make a career last as long as you play...not just 'til you finish 100%. I want to race the 24H of LeMans 5 years in a row and have a record of how well I did in all 5 years and so on. I want rivalries and friendships to be implemented. That's what racing's about.
As for the customizing, paint would be fun, but exhaust tips, neons, tinted windows and hotrod tape, I believe those belong in NFS as well. I think the drift style is cute in GT, but anybody who wants to be fast on tarmack knows it's useless.
The tracks, I like to see more real life ones, but I love the made up ones like El Capitain...still one of my faves.....
And, well, Porsche, Ferrari and Lambourghini should be there... Imagine recreating the LeMans Steve McQueen movie.....I'm salivating just thinking about it. also More cars on track...
I'm done...I think...