That's what 3PP covers. WotC is building the basic game and has their focus. They are currently successful.It'd be fair to say that there are styles of play, sub-genres, and character & campaign concepts that are unsupported or inadequately supported, by the standard game
My advice is look at the 3PP for those niche sub-genres, styles of play, and character concepts. Then pick one a person likes and buy it. Supporting those products demonstrates a market for those products. It's the vote-with-your-wallet method and those designers are certainly going to appreciate the support for their work.