Again, even in individual genres, all the changes didn't happen at once. Someone took the base (Wolfenstein 3D, 1992), added to it (Doom, 1993), refined the details (Duke Nukem 3D, 1995), and used more advanced technology (Quake, 1996).
There's an extremely old saying made famous by Isaac Newton, who said, "If I have seen further, it is by standing on the shoulders of giants.". That is, we're only building on the previous discoveries made; ironically, Newton's quote is lifted from another academic from centuries before. The same applies to game design. The definition for "next-gen" is so vague and meaningless it might as well not be used; case in point, the limited weapon system (or two/three weapon system) that Halo (2001) made famous and copied by 99% of all FPSes from then on was originally used in Rise of the Triad (1995), and even that is just an extension of the weapon powerup systems in games like Contra (1987) and Smash TV (1990).
I don't consider there to be any definition for "next gen". It's an empty phrase; the better side of propaganda. Next gen as we know it is defined by the power of the technology, but the technology only defines the height of the ceiling. We're only limited by our creativity; there are plenty of Retraux games out there that show what we can do with technology from yesterday with the knowledge from today. There will always be new additions to every genre; someone new will always come along and add another brick to the house, but it's never going to be something that you can define as "next gen", which as I've said before, is a meaningless term.
For example i think as the unification of damaging stats as a "next gen" thing, where mages and fighters use the same stat but applied in a different way ( like saying attack damage and ability power being one, but used differently) so you don' t have an halved list of items, with some retaining an identity exclusive to some roles.
Ironically, this is a Gen 1 concept in League (Pre-beta, Pre-season 1), having long ago been ditched for obvious reasons. It used to be that Attack Damage affected only basic attacks, and all skills (including pure AD champions like Sivir and Garen) scaled off of Ability Power. Then it was changed.