Ohh good question, for me it would have to be not excatly a mod but a really easy mod program that lets you copy exisiting models from other games and paste them into the game and it automatically changes the graphics to GTR EVO Standard.
NASCAR. I tried ARCA and...meh, the graphics looked horrible, the handling was a bit off and yeah, just not fun overall. And we all know the NASCAR games by EA are absolute crap. So a good NASCAR mod would be great for Evo.