I've been picking up some of Exactrail's new FBOX Hy_Cubes. These are great cars all the way around, the molding, lettering, body mounted coupler, metal wheels... they are some of my favorite rolling stick. But there is always room for improvement. For my newer cars I like to hit them with a dusting of a diluted L&N Grey, 8 parts isopropyl alcohol to 1 part paint. I also take a micro brush and apply roof brown to the sides of the wheels. This tones the car down and gets rid of that "new out of the box" look.
The other area that I've been upgrading on my cars are the crossover platforms. FVM and Athearn have been including these on many of their new cars. I like them for a few reasons: 1) They look more prototypical. 2) They close the visual distance between cars, making our oversized N Scale couplers look a little smaller. and 3) If I take my cars to a FreemoN ops session, I can more easily distinguish my cars from everyone elses.
This upgrade is so easy it is crazy. I use BLMA 403 platforms. You get enough for 10 cars in each pack. I drill a #80 hole directly on top of the center of the molded on platform, put a little CA on top of the molded platform and on the stem of the etched platform and stick the two together. No painting is needed as most platforms are bare metal. Make sure to put the platforms with the notch on the end with the brake wheel.
Between the weathering an the platforms the cars appearance improves and an otherwise great car gets a little better.