I’ll make a bet you see rowing in gta 6 before you see it in zwift.
Keep an eye on the Rockstar games job adverts now development has started.
For a company that runs a cycling team and has sponsored a crit series, enveloping something like say… gtbikev much more formally into an existing online world and support system would make alot of sense given the apparent market valuation of zwift no? 
I prefer Zwift to be a training program rather than a game. GTA might be fun visually but is the biking part of it actually of any use for realistic training?
gtbikev is a decent mod that lets you group ride etc. using gta5 maps (run TR if you want).
Its very hard to see rockstar games having done all the hard work of creating/supporting the online world + physics etc. not running a small development team for gta6 given zwift has a post money valuation north of $1B.
I haven’t tried it myself yet (crappy PC) but it’s actually meant to be pretty good. Shane Miller (and plenty of others) has a video on it.