This is one of the first things I did when I implemented my own Bing proxy, although I did the site boosting/downranking/exclusions and rewrote URLs (e.g. www.reddit.com > old.reddit.com) myself since I didn't have any users, but the plans was always to give users the ability to control this.
I later learned that it's actually against Microsoft's Bing Search API use and display requirements (https://docs.microsoft.com/en-us/bing/search-apis/bing-web-s...), which states that you're not allowed to:
> Modify the results content (other than to reformat them in a way that does not violate any other requirement), unless required by law or agreed to by Microsoft.
> Reorder, including by omission, the results displayed in an answer when an order or ranking is provided, unless required by law or agreed to by Microsoft.
> Display content that was not included within any part of a response in a way that would lead a user to believe that content is part of the response.
And there are many other restrictions imposed upon you. I'd argue the reason you see almost zero innovation in this space (the only thing I can think of is !bangs) is because non-Google/Bing(/Brave) Search Engines have very little control over the received data, and with the little control we do have then there are many restrictions that prevents us from making these 'innovative' features.
Based on my pessimistic reading then we're not allowed under any circumstances to merge API results from multiple search engines (e.g. Bing + Yandex + Google), and for every site we want to exclude/boost/derank/rewrite sites/urls then we need explicit permission from a Microsoft representative (which would prevent us from implementing this kind of feature). We're also not allowed to display third-party ads on any pages that displays part of the API results (which is funny because I feel it's near impossible to join Microsoft's Advertising network even though my site in my opinion have a good purpose and provide a high quality UI/UX experience - https://ask.moe).. I've later switched to Google's Programmable Search Engine and even though my site has been approved by Google as a non-profit, as well as approved for AdSense, then they will only display ads in the search results if I turn off revenue-share.
This industry desperately needs legislation that makes it possible for everybody to obtain access to Google/Microsoft's crawled data, and this access should not be limited to expensive APIs that make it impossible for anybody besides Google/Microsoft to do any sort of innovation.