As far as roadcars, there aren't a ton of new cars they'd have to add to keep GT7 up to date. Most manufacturers have two or three new, relevant sportscars. Some don't have any. Some have multiple trim levels of the same car which can essentially be one development car that is tweaked.
Two or three new, relevant sports cars...
if PD was up to date on their offerings for GT6, and that's ignoring the more pedestrian offerings the company might have (which has long been a talking point for the GT series: the "normal" cars). As just one example: BMW's modern offerings amount to the M4. The rest of the M offerings aren't found, unless we go back a generation to the E92 and E60. Then there's the 1-series M Coupe, which has always seemed like a strange omission to me considering the 135i is in. Perhaps the replacement, the 2-series, could be added. There's no road-going version of the current Z4, either.
Come to think of it, a lot of the German marques suffer: Audi didn't get a single road car in the move to GT6, so their newest offering is the '09 R8 V10, with no S or RS models from the last 7 years. Mercedes had that odd Red Bull blog snafu, where the E and A AMG products were mentioned for inclusion, but then removed. They too didn't receive a single road car for GT6. Not a single instance of their 5.5L turbo V8 in any of the many vehicles it's gone in, and it's now been replaced by the 4.0L in the new C and AMG GT. VW arguably got the best treatment, though not for GT6 either; they got the Scirocco and Golf R as DLC near the end of GT5's run, but it gives them a little more modern representation.
Ferrari has updated their entire lineup since GT5 - as it's another make that didn't see any newer models added for GT6. The California would be the easiest one to update, what with the new T spec, and the 458 Italia is only
just bowing out now for the 488 GTB, but they're still missing a huge swath of the V12 lineup. Lambo really got some of the best representation in this game.
I suppose if they added a new substantial manufacturer (i.e., porsche) that could be many, many cars. Or if they decided to expand Ferrari's historical offerings, that would likewise be a pretty big number.
Or Ford's, really. Because of PD's reliance on Standards, most of the Japanese companies are represented pretty well in the historic front. But venturing off the island, a lot of companies are missing a lot of big names from their past.
Tying this all to the topic: I'd love to see PD plug these gaps over bringing in more race cars. It seems like a safer bet: I'd imagine those more hardcore sim-racing fans that flock to race cars (and PC sims) won't be won over as easily with a few new race car additions as the general casual gamers will be won over by an epic road car list.