Robin Rams on YouTube uploaded a fan GTA: The Trilogy trailer. In his video he used the Vice Cry: Remastered mod, some shots from mod for GTA San Andreas on Unreal Engine 4 and other fan works.
Reported PC requirements for Grand Theft Auto: The Trilogy – The Definitive Edition allow us to conclude that it will improve more than just textures. If the official trailer from Rockstar Games will looks anything like this, it will be a success for the developers.