Look in the Rumor and Speculation board and you will see heaps of threads about what people want to see in GT4. These things probably answer your question.
For me I would spread the prize cars out more ie I would not have a Vitz available at so many series.
Maybe use the colour options on the prize cars to spread them out.
I just found that by the time I got half way through professional I had all the prize cars.
I would have a wider variety of endurance races eg FF, MR, FR, 4WD. Basically many of the race series would have a corresponding Endurance.
I would also make S license and Vitz pro a little easier but when you get to 100% it would unlock a difficulty option that affects every race ie you would be trying to get 100% again but it would be a little harder. Maybe this could include HP limits.
Would include American LM cars if licensing permitted and would include race tracks like Mosport and Sebring.
Either beef up rally or get rid of it eg more tracks maybe including a dirt oval and point to point tracks.
On the oil change issue here are my views:
1. In motor races do they change oil mid race ? I think not but could be wrong.
2. In series they do change oil so this should be an option. If they allow damage in GT4 then maybe they could have a time limited thing like in the rally games where you have 60 minutes to allocate to various repairs.
3. If oil changes are added to pit stops then I would speed up the AI cars anyway because otherwise it would be too easy.
There are a lot of subjects there on changing things in GT3.
