You're the new type of graphics whores that I don't like. Putting graphics in front of gameplay. Just because it's cross gen doesn't mean it's going to be a bad game. It's just as bad as people not buying Watch Dogs because it's 900p and not 1080p. What the hell is wrong with people?
The idea is that these open-world games, especially first person open-world games, would probably reap more gameplay benefits from being based on systems with more RAM and more powerful processors. A current-gen-only Far Cry could potentially have larger environments with more dynamic things going on. The original Far Cry was never faithfully ported to the consoles of its time (Xbox and PS2) because of hardware constraints. Rocksteady seems to think the gameplay they're trying to do for Arkham Knight isn't possible on PS3 and 360. CDProjekt RED has already confirmed Witcher 3 would have to be a different game to be playable on PS3 or 360.
Diminishing returns may've kicked in harder than ever. Not necessarily visually (though that's significant relative to prior generations) but gameplay ideas. There probably isn't really that much more you could do on PS4/XB1 that you couldn't have done on PS3/360 already, so just murder the graphics and it fits! Though there's still stuff like fitting the Bat mobile into Arkham City gameplay, so it leaves me to wonder what may not be doable in these games because they want to keep PS3/360 there.
I think gameplay-related diminishing returns started to kick in sometime during the era of PS1, N64, and late 90's PC games. Most of what we've seen since then has been variations on genres invented around that time with more gameplay refinements and a few ideas people just happened to think up later. I think a massive chunk of PS3 and 360 games were mechanically possible on PS2 and Xbox, just with less detailed graphics. Hell, a lot of those games are just successors to late 90's and early 2000's PC games but actually have LESS complex gameplay.
Open-world games are probably one of the big exceptions depending on how much detail and how many factors developers put into the environments. I just listed some examples in the response above which are games with huge environments.
I don't think they get any engine updates, but since it's derived work they still have to list it and probably pay some kind of fee.
It's the same way the Splinter Cell team's LEAD engine is based on Unreal 2 and Call of Duty's IW engine is based on Quake 3.
