I've been thinking about this for a few days now, while I got lost in modding and painting up cars in Forza. I can't seem to push myself away and get back to Gran Turismo, I'm totally addicted to creating race cars, and sadly, GT doesn't offer this. Yet.
When I create a race car in Forza, I'm looking at it from the perspective of either a serious SCCA Pro level car, not just a plate of street rice, or a full blown Touring or SuperGT class car. I treat each car as if I were the owner, wanting to make a big splash on The Speed Channel and impress the sponsors, so I try to come up with a professional looking paint scheme, maybe a little tricky but nothing like a Fast n Furious ride. I also use decals from serious manufacturers of high caliber, and a sensible number of them in a tasteful arrangement, not a car plastered bumper to bumper with two of everything. I choose one or two to be major sponsors, and on occasion I'll create a sponsor decal which Forza doesn't offer like Mobil 1. I create a number and make up a badge for the car, and a named driver - usually, not all cars have names on them yet. I have about 10 virtual racers in mind, some father and son teams with liveries which share some similarities.
I've been studying manufacturers and which ones different real life race teams chose to build into their cars. I've researched the racing parts makers like A'PEXi and HKS to see what they offer, so that I know what packages I can build a car with, and how many different companies I have to go to for a complete racing package.
I just finished a Ferrari Enzo a bit ago, with a basic red and blue motif, but it looks pretty snazzy. I chose major components from FK (exhausts, intake, mirrors, seats, suspension), Magnetti Marelli (alternators, electronics, fuel systems, ignition, engine components, sensors) and Prodrive (transmission, differential), so their decals are prominent. I have smaller ones I chose to round out the car from Eurolite (lights), Ferodo (brakes), Fram (ignition, oil systems), Ractive (aerodynamic body panels, struts, suspension coils), and Rays racing wheels - I had trouble getting Fikse to fit anywhere legibly.

Ferrari stickers are here and there, as well as a Forza badge. I located the number badge on the rear wheel housing panel because visibility is bad everywhere else. It may be out of racing spec but this is for fun anyway. When I come up with a driver and nationality, his name and a national flag will go on. I'm still fussing with some league sticker ideas like IMSA, probably go with that.
I should snap some pics when I get a chance. I have about a dozen or so serious race cars done up with reasonably cool liveries.
So about racing modification and sponsors. I just did a big post about online play vs offline, and how I saw it working out as far as progress and rewards, as well as leagues to join, so some of that is available
HERE.
I'm beginning to wonder if there might be two stages to what we can do with our cars in GT5 as far as modding them up. In the serious leagues like Super GT, I'm not sure what sorts of body work can be done on those cars. In the semi-pro world like America's SCCA, I'd imagine you would approach building a race car like you do in Forza, where you select body components from established racing manufacturers that help your car cut through the air while providing downforce to help you grip during turns. Kent in his wisdom mentioned modification packages, which I had been contemplating from all the great tuner cars out, some of which made it into GT4. I bought up the lot of them, but have yet to take many of them for a spin because of my Forza addiction.
But with the big name racing leagues like ALMS and Super GT, what exactly are the options a car builder has as far as body work goes? Are there a range of choices available? Or are there league approved kits that you get? If that's the case, in the big league racing mod area, we might find that we get those black cars, which we would then customize internally with whatever components we want. Whatever the deal will be, I don't want to enter a race with the Raybrig 100 NSX, as much as I love that car. I want to create an NSX or whatever racecar from scratch, with my own paint job, my own number and my own name. Along with that, my own sponsors and my own decals when I enter a race. When I have been going through Forza the second time through, I mod and paint up my car - if the game will let me - to create my own race car. I want the same options available in Gran Turismo 5.
As for the leagues and sponsors, this will be cool. I can see Polyphony giving the racing community the opportunity to be like the real racing world, and giving us the tools to create our own racing leagues online, with our own rules and regs, as well as logos and decals to go along with them. And then there's sponsorships. Surely there will be some sort of pecking order, where drivers will have to prove themselves in order to win sponsorships. How that will be done, I don't know. Maybe the data from your success in GT5 will be used, maybe you'll have to take an online race test - license test anyone?
Whatever the deal is, I'm hopeful that Polyphony will give us an awe inspiring gift when Gran Turismo 5 goes online around the planet, and we find the racing world of our dreams. Now I have to take a nap so I can go Christmas shopping in a few hours.

Maybe tomorrow I'll start on getting pics of some of my cars.