Arguably already happened, via notifications. They've long allowed third-party apps to send notifications that are actually ads, and a few first-party apps (most notably wallet with the F1 movie ads last year) have started doing this too. Those made it to the lock screen, and if you happened to dock any of those apps, the obnoxious red notification dot would induce you to click on it.

Any app that did that to me lost its notification privileges permanently and got uninstalled if it wasn't a built-in app, but I suppose Apple could start directly injecting unavoidable ads too.