I'm sure they will expand on gameplay but there's no indication they'll come close to scratching the surface of PoE's depth, and I don't think they're trying to. D4 is supposed to be a simpler game with a lower barrier to entry that is more accommodating to casual players. You can't get a whole lot out of PoE as a beginner without third-party resources, there's certainly a tradeoff to its many layers of complexity.

D3 and D4 are games made for people in their 30s who grew up playing D1 and D2 and now only have a couple of hours a day to play, and that's fine