New thing I would like to see is realistic representation of transmissions, no more using a stock transmission with a car engine power boosted 200+ hp beyond what it was designed for and no repercussions. Either you will be warned that the engine could likely cause your transmission to fail at inopportune times or you must install a higher spec transmissions, this whole car can drive with stock tranny crap is ridiculous, you can drive but racing is out of the question. Unless your car already sported high HP then maybe the transmission can run stock, but those trannies are usually higher spec than everyday econoboxes anyway.
I want the livery editor to be more robust, in fact I want it to be more like a custom shop where you can do as you see fit. Just like how in West Coast Customs paint shop, they can pretty much customize the pain scheme, and paint ANYTHING on the exterior. If for instance I wanted to have my rims colored in a style that I came up with, let me be able to put that on it. Not the just paint the whole rim approach that FM3 is known for, but much more intricate(two-tone, color face/side for the 3 dimensional styled rims). Just hand me a spray gun and let me paint it how I see fit, this would make the online community pretty diverse also give people real flexibility in making creations all their own. The body paint should be done in a better way, I say since GT5 cars are built with independent panel parts, we should be able to paint per panel(quarter panel, door panel, etc.), from there they could allow option to take said panel then have the option to make the scheme of your choosing by allowing it to be on the fly manipulation of how much of the panel the color should cover and such(like using vinyls, but this is just paint). I hate using vinyls to make simple two tone color schemes, even worse for multitone, that stuff should be paint jobs. In fact I wonder if PD will allow use of player created stencils that can be uploaded and applied to cars then painted just like in adobe? That would be awesome. I'll be back with a few more ideas.