The Apple developer community is excited about a rumor out of Bloomberg News. The original article claims this is about a new cross platform API that allows developers to write one app that works on iOS and macOS. I doubt Apple will produce such an API, and that this rumor is a misinterpretation of something else.
Right now, you can combine the sale of iOS and tvOS apps in their respective app stores so that buying one version of the app buys the other version of the app. You buy the app on your iPhone for a game, and you have it on your Apple TV too.
I think the expansion of this system is behind the rumor. Apple will soon allow developers to link their macOS apps to their iOS and tvOS apps so that buying a game, or social network client, or TV service app on one platform means you bought that app on all your Apple devices.
Developers will continue to develop separate apps using Apple’s separate iOS, macOS, and tvOS SDKs.
I think it will be optional. Developers can choose to charge the customer for each platform, allowing paid-up-front business models to continue. This is for free-up-front business models. HBO wants their apps on all the devices, as long as you pay for the subscription; games that are monetized through in-app-purchases would like their apps on all platforms.
I think this because Apple clearly thinks its platforms are different and should have separate APIs. When the company developed tvOS, they branched away from iOS. They could have kept the two OSes as one, coming up with TV interactions to work within the confines of iOS. Maybe they could have made the Apple TV remote a minimal iPod touch, where the big TV screen on the wall is merely a mirror of the remote screen. Then iOS would have been a perfectly serviceable OS for the Apple TV. Instead they made a new UI for tvOS, and gave us a new OS branched from iOS that gives developers tvOS unique API.
Apple acts like it considers iOS and macOS are different, targeting different UIs and different uses. They are not likely to abandon that stance, and produce a new complex API that allows one app to run on both platforms.