Ok, this will have probably been mentioned in the pile of posts with suggestions; but, to those of us that went through this kind of progression in the game in the past probably will understand this better than any:
Bring back the license requirements BACK to the career progression and (might anger the ones who like to just jump right into an online instead of actually going out and playing the game through itself) put in some kind of license requirement for online play.
It makes a lot of sense (to me, and maybe others might agree) that if you actually put the effort in with a set of benchmarks in front of you, you just might actually have a better feel of progressing and improving as you go along.........and you are able to adjust much better than having jumped in with little to no progress whatsoever and expecting to do quite well.
I could see having a level requirement necessary to start a particular license and opening up progression to the next license (no problem there); but, to me, with the way it was built in GT5, the licenses virtually have NO impact on gameplay progression and (to me) online progression, also. All it seemed to be was a glorified way of just earning some extra cars without having a career progressing impact (no, extra exp doesn't mean diddlypoo, imo).
Also, with that said, adding in an extra license set (progression as follows: C, B, A, IC, IB, IA, S) in order to add a Time Trial component as the IA licensing before progressing onto the Super License tests would make an extra challenge as it was a fun challenge to try and do a fast hot lap cleanly knowing one bad move would force you to start all over again. I liked how the Super Licenses were set up and having a Time Trial component to help progress you into a 'Driving School' graduation (as that is how I deemed the Super License test in GT5) would seem to be a fair and excellent way to progress.