There's the Big Five, who would probably dominate.
But there are also many smaller publishers and small presses who might make their presence felt.
The rules would need to define "publisher." Some publishers have multiple imprints, each with its own line and editorial staffs. Are those all grouped together, or do they compete separately? Random House recently acquired Penguin to form Random Penguin (no, they are not calling that, but they should). One corporation, but still two separate imprints and editorial staffs. And even before the merger, Random House included Bantam Spectra, Ballantine, Doubleday, Del Rey, Dell, and more.
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But there are also many smaller publishers and small presses who might make their presence felt.
The rules would need to define "publisher." Some publishers have multiple imprints, each with its own line and editorial staffs. Are those all grouped together, or do they compete separately? Random House recently acquired Penguin to form Random Penguin (no, they are not calling that, but they should). One corporation, but still two separate imprints and editorial staffs. And even before the merger, Random House included Bantam Spectra, Ballantine, Doubleday, Del Rey, Dell, and more.