As stated before, Rockstar separated their games into three different universes. These universes, according to Dan Houser, do not interact with one another. In other words, Claude - from the 3D universe - cannot randomly appear to help Niko - from the HD universe. They are different places. So all of these rumours that claimed that CJ, Tommy Vercetti, Tenpenny, etc. would be making appearances - they're dead wrong. Rockstar even clarified that there would be no appearances of main characters from GTA IV. This means you cannot expect to see Niko or Roman (mostly because Roman had a 50/50 chance of dying) or Packie (who Rockstar several times confirmed will not be in GTA V). Who can you expect to see? Probably somebody along the lines of Bernie Crane (AKA Florian Cravic) or Yusuf Amir. But don't have your fingers crossed hoping to be seeing Johnny Klebitz or Luis Lopez. You will be disappointed.