We need a commission cap on all platforms that do over $1billion in annual sales. The Apple vs Epic trial revealed 80% margins for Apple on the App Store:
https://9to5mac.com/2021/05/01/apple-runs-app-store-with-78-...
A cap of 10-15% should do it.
Like on iOS, if there was actual competition for app stores, Apple would face pressure from a competitive market to reduce their commission percentage. Developers will not want to put their apps/games on Apple's store if they continue to charge outrages fees, and they have alternative, realistic distribution options.
However, I don't know what regulation would look like there. Even if Apple is forced to allow third-party stores, it could still end up like the browser wars, where one company manages to secure a monopoly anyways. If we can't even fix the browser situation, I don't have high hopes for the appstore market.
Countries around the world already regulate pricing on things like Credit Card transaction fees. This should be no different. In Australia for example:
"Each scheme's average interchange fee, weighted by the value or volume of transactions in each interchange category, is required to be below a benchmark of 0.50 per cent for credit cards, and 8 cents for debit and prepaid cards."
https://www.rba.gov.au/payments-and-infrastructure/review-of....