Apple may well create a 'sandboxing' mode that allows unsigned apps to run without access to system processes and user data. They will probably also distribute alternate app portals via the App Store itself meaning Apple gets to vet each one. I'd be amazed if they offered anything close to Android's open nature.

It doesn't really matter of course because less than 1% of Android users bother with a 3rd party app store, even when they come baked in like on Samsung devices. Apple can offer these things but they know nobody is going to use them (beyond people looking for emulators)