As most of us know, the person that played Niko left Rockstar after GTA IV was made. This is mentioned by the wanted poster of Niko and how Lester says "an eastern European kind of guy making moves in Liberty City, but he went quiet" in GTA V. As Niko was one of the most hardcore chariters in GTA and one of the best chariters in GTA, most wonder "where is Niko". As he is a parent model in GTA online, this may be the last we see of him. If Niko were to come back, it would have to be before the HD universe ends, because when a GTA universe ends, all things in that universe would cease to excist in the new universe, (as demonstrated by the end of the 2D and 3D universe.)