The special content doesn't make sense as GTA V is set in a different era than GTA III so Claude cannot appear even if just as a parent for a custom character.Niko may be plausible to appear but then it makes us question GTA Online's place on the timeline as in 2013 if we assume Niko had a child in 2009 (The earliest possible if Kate became pregnant some months after the Deal ending) he would only be 4 years old making it impossible to be his child.A retcon is clearly possible but unlikely since in GTA III Liberty City was entirely different and, in 7 years from 2001 to 2008 (Where GTA IV is set),it would be impossible for it to change that much.
It doesn't matter. It wouldn't even if multiplayer events were to be considerd canon, which they likely won't.
Using any GTA 4 character wouldn't make sense nor would using any character from GTA 3 or before, all they are are Special edition incentives, not many people would buy a special edition with unknown characters over one with known characters in. It's just Rstar's way of boosting special edition sales, it doesn't mean anything.