If Sliders has taught us anything, it's that people from one dimension often have doubles in parallel universes. So a version of any character from the GTA3 universe could exist in the new one, and would technically be a new character.
Not saying they would actually do that, but it's fun to think about how things might have turned out for certain characters, had their world been somewhat different.
that suggests that characters from 4 will be returning...
R* has pretty much confirmed this, they just haven't said who.
The GI artcale I read said none of Main characters (Niko,Johnny and and Luis) would show up in 5. Probealy also goes for characters like Kate and roman won't be there since R* would want to leave the ending ambiguous unless there bold enough to confirm the ending
Although good money would be on Packie, Brucie or another minor character will