My best suggestion if it's to be truly neutral: Allow all search engines with more than some small number of European users (say 10,000 monthly active users fo

My best suggestion if it's to be truly neutral: Allow all search engines with more than some small number of European users (say 10,000 monthly active users for six consecutive months, or pick a higher but still broadly attainable threshold) to apply for inclusion for free. Randomly pick four or five to show on the main screen each time, in randomized order, without treating Google specially. If necessary, have one special slot that cycles randomly among all the top several search engines in Europe, and exclude those from the other slots. Have a "More options..." link which shows every search engine who has applied, again in randomized order. That's it. But yeah, I don't expect them to be that neutral unless forced.