Hmm. Separate thoughts, first on designers I’d expect any reasonably fair observer to acknowledge as important trailblazers whether or not the observer happens to personally like the results. At the top of the list, Greg Stafford, whose name here should be set in large type, blood, italic, and blink. After him, people like Jonathan Tweet and Robin Laws, and colleagues like Rob Heinsoo, Chris Pramas & Nicole Lindroos, and the aspirational Erick Wujick and Jenna Moran.
These last two inspire a lot of folks to go fight on whatever direction one may be going, without many efforts to go in directions they seem to have been to been going themselves. As George R.R. Martin said of Howard Walrop, not so much “damn, I wish I’d that idea” as “what the hell was that?”
Sub-thought: anyone who’s been a senior designer on an edition of D&D is a significant influence on the field, whatever else they do or don’t do.
I tend to think of “modern” rpg design as a funnel with its mouth in the ‘70s, widening some in the ‘80s, a lot more on the ‘90s, and encompassing much of the field after that. Yes, this explicitly puts the general milieu of modernity before the Forge. I always saw Thor best things as pulling existing elements together in new blends and then interacting on the results.
Like others, if I had to pick one diagnostic feature, it’d be intentionality. Specific, not just knowing what they intend to happen during play and a result of it, but TELLING PLAYERS ABOUT IT. The second part is where earlier games tending toward modernity are most likely to fall down. After that, I’d put traits thought through for their purpose in this particular game, and player-customizable as much as is suitable (within specific limits like cultural keywords in HeroQuest/QuestWorlds, templates in Star Wars et seq, et very cetera).
There’s kinds room within this framework for OSR games and the like, which have a degree of deliberateness and overtness progenitors lacked. I could make an argument for them as postmodern, but I’d have to quote Umberto Eco’s “Postscript to the Name of a Rose” and then exposit from there, and it would anyway get Argus with over whether OSR revisiting is ironic. So nah.