It would definitely need to be something more than a straight port, even if the port was done well, in order for people to take notice in large numbers.
Aesthetic improvments (e.g. HD resolution, textures, reflections or higher quality audio) and achievements/trophies are a good start. Perhaps there could be some sort of online leaderboards function or something along those lines, to add a bit more spice in order to help people think about buying it, especially when they probably already own the original games along with their previous gen console.
That said it's starting to sound like a fair bit of additional work (i.e. money) which I imagine Rockstar would rather pour into current projects.