Bingo. This is exactly what I recently proposed for Twitter. The idea of user defined "filter-sets" where the user chooses what they want to be exposed to. And even further, I proposed the ability to name those filter-sets and share them with others. So if a given user is, for the sake of argument, a "raging Liberal" they can choose a "raging Liberal" filter-set. Likewise, a "raging Conservative" can choose the "raging Conservative" filter-set, a Libertarian can choose the "Libertarian" filter-set, and so on. Much like your market-place idea mentioned above.

"But, but, what about filter bubbles / echo chambers???" one says. To which I say "it's not a problem I'm trying to solve, or particularly interested in. I'm not even sure that's actually a problem per-se to begin with." Seriously, the world is too big, there are too many people and too many ideas, for any one of us to accept being exposed to everything that's "out there".

Filters are a good thing, in fact a necessary thing. Of course we'd like to think that in a well educated, civilized, rational society, people would choose to make some effort to gain exposure to new and contradictory ideas for the purpose of expanding their minds and adapting and learning and what-not. But be that as it may, you still have to accept that some people will choose to filter out things to a lesser or greater degree than some others, and that some people will choose to filter things differently than you will. And that's OK.

Until your list of "popular filters" include ones such as "hide jews" or "hide blacks", following clamours for a central authority to ban those filters, or to shut down the entire platform.