EDIT - My apologies for the huge post, I swear it seemed so brief in my head..
This is a list by the way, but I've posted it in paragraph form to make it easier to read.
A realistic calendar - i.e. the game starts on January 1st, not 'Day 1'. There would be several clubman/entry level events going on every day (although you could still only enter 1 per day, there's simply many to choose from on any particular day) whereas some of the more 'professional' events only come around a couple times a month. I think this would be a good idea, as PD could then make some of the more expert races (a 6 hour Nurburgring race, for example) only come around once or twice a year, and the prizes and prize cars for certain races would be that much more valuable if you theoritically had only one or two chances to win them per in-game year. And for those of you who will quote this and whine, there's always the good ol' standby of saving before the race in question and therefore having as many times to complete it if you wish to, and I use this word loosely, 'cheat'.
Have certain series going on throughout that calendar year that are replicas of real-life series. Obviously, acquiring an ALMS or DTM license would be great, but unnecessary. You would have to pay an entry fee to join those series, and then have those races coming up along the calendar year (as opposed to the way it was in previous GT editions - Race 1, followed immediately by Race 2, followed immediately by Race 3, etc.) In other words, and I'm completely making this up here, let's say your a few in-game years deep into GT5, and you've got a couple million in the bank and a beautiful Audi R10 sitting in the garage. You could then go and pay (again, for example) 10,000,000c to join the 'Endurance Championship'. You're free to do whatever you like as the in-game days pass, but on March 19th there's a 2 hour race at Infineon you now get to compete in. A few months later, on May 2nd, there's a 6 hour race at Monza, etc.
Winter/off season testing dates for those championships.
Would be great to have the occasional non-championship, no points ('exhibition') races, where you could (just for fun) enter a 3rd car, or enter a recently-bought car in a different class to see if you like GT2 better than P1, etc.
It would also be neat if companies (whether real of fabricated by PD) offered to buy the team from you for some exhorbitant amount, like 25,000,000c if you've been particularly successful).
These series could become even more realistic if, from year to year, the races on the calendar changed. One year you might find that the dreaded 12 Hours of Motegi has been dropped, but 6 hour events at Laguna Seca and Road Atlanta have been added.
Again, going for realism, even the AI controlled teams could drop out from time to time, with different teams coming in as well. Maybe the first season you sign up, there are only a couple of Corvettes and a privateer Aston Martin in the GT1 class, but as the years progess, Saleens, Vipers, Ferraris, etc. are added to the series.
Include pages for all series like my ideas that show records full records of past seasons - in short, essentially make Wikipedia-esque stat pages you can go over at any time, including finishing results, times, fastest laps, DNQs, DNFs, etc. of all the races in that series' history. It would certainly bring the nostalgic factor back up, as 12 in-game years down the line you could go relive the rivalries you/your team developed over the seasons in that series.
In accordance to the above suggestions, certain races on the calendar include prizes that are more than just a few hundred thousand credits - invitations to 24 hour races, as just one example, races you could only ever compete in (and thus only ever win the prize car and be able to hit 100% completion) if you joined the corresponding yearly race series in the first place.
You could also take the above idea a bit further and have the ability to enter 2 cars (provided you've already bought/won 2 cars) and therefore have a team to race in the following season. One of the cars would be controlled by the AI during the races, and consequently we could have a fairly realistic representation of ALMS/LMS/FIA GT (and so on) with both Driver's and Constructor's Championships up for grabs at events throughout the year. I'd also imagine that implementing different classes to race series like this wouldn't be too difficult - Maybe intitially you just have enough money to enter a single, 'privateer' Ferrari F430 into the GT2 class, but after a few seasons (and your activities outside this partiucular championship) you might have enough to be able to enter a second car, or even move up into one of the other classes available in that series. You could also make it somewhat model specific (i.e based on power, weight, production status, coupe/open top, etc. of whatever car you plan on entering into the championship, the game automatically decides what class of car (GT2/1, P2/1 to use a real-life example) your car fits into. You could also have the ability of reducing your horsepower to take a GT1 car down to GT2 if you feel the costs are getting too high.
This brings me to my next suggestion, and my apologies for make a post as huge as this one has been. The GT4 system of entering a race (click on the race, start the race) is fine in GT5 for clubman types of racing, but if things like my above suggestion were implemented I'd want a bit more than just start and enter. For example, say you entered a car into the GT1 class in the above suggestion, and there's a 6 hour race at Suzuka scheduled to begin on August 12th (remember, this is done using a real calendar). You have a 2 hour testing session on the 9th, and another 2 hour (real time) session on the 10th. During these, to keep it all as realistic as possible, a set amount of time would be taken away for modifications done to the set up of your car. Then on the 11th there's qualifying for the race on the 12th. Again, to make it realistic, each series (i.e each incarnation of the above suggestion) would have different types of qualifying, different regulations, different rules (in addition to the obvious differences of cars participating, tracks, series length, etc.)
You would need to acquire specific licenses (not just B, A, IC, IB, IA, S) to enter these races.
I would also like to see an OPTIONAL pop-up on screen (eg. a bar running along the bottom) of the current race standings, and (again, the OPTION to) show real-time differences of how far you are behind the car infront of you and infront of the car behind you. (As opposed to being stuck with just a 2 second notice of how far behind the rabbit car you are each time you pass a sector, although keep that as one of the options).
That's all for now
. I've got half a hard-on just thinking about it.. a man can dream, can't he..
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