Vehicle models that are in no way shape or form Xbox assets. They're incredibly high quality models that in no way constitutes "simply using the Xbox assets". There's also numerous texture mods that, again, don't use Xbox assets.
Even if we ignore that there's still...
GTA United
GTA State of Liberty
GTA SA:LC
SAxVCxLC
GTA VCS PC Edition
...just to name a few...none of which are "simply Xbox assets"...
It already is a better version. Among other things it actually runs on Windows 10, adds the ability to control the camera, adds numerous bug fixes, uses OpenGL / DX9, supports PS2 / Xbox rendering features out of the box, has audio that actually works properly without having to fuck with the miles sound system dll to use OpenAL, supports arbitrary resolutions and aspect ratios, completely removed engine limits...etc, etc...
It's literally better in every single way...and I can guarantee it's better and more faithful than whatever the fuck Rockstar is cooking up. Among other things that were coming was a proper decoupling of the physics and framerate, allowing the games to run flawlessly at high framerates.