(Disclosure: I'm the Eng. Mgr for the Flutter project at Google.)
In general Flutter takes the position that Flutter should not limit your choices and your apps should be able to do anything the phone can do. Flutter apps are just iOS or Android apps, you can always open up the ios or android sub-folders and write as much Obj-C/Swift or Java/Kotlin code as you like.
Flutter's core engine/framework are all about providing portable UI, and leave everything else up to "plugins" (the opposite of say the Web were everything is baked into the core platform). Lots of plugins exist today, but there are always more to write:
https://pub.dartlang.org/flutter
https://flutter.io/developing-packages/
https://flutter.io/using-packages/
Responses to your specific asks:
Showing arbitrary native views inline with other Flutter content is actively in progress: https://github.com/flutter/flutter/issues/19030 (already possible to show views to the side of/on-top-of a FlutterView of course).
Full inline Google Maps is similarly in progress: https://github.com/flutter/flutter/issues/73
There are several community authored plugins for geolocation e.g. https://pub.dartlang.org/packages/geolocator.
AR is not something I've seen any work on yet, but it's always possible to throw up a full-screen ARKit/ARCore view using ObjC/Java your otherwise-Flutter-built app.
Flutter includes BoringSSL as part of it's runtime, some encryption APIs are exposed, there is probably more for us to do here. There is also pure-Dart crypto, e.g. https://pub.dartlang.org/packages/crypto and of course always possible to write your own wrappers around iOS/Android APIs (which someone may already have done too).
Audio/Video plugins exist today, e.g. https://pub.dartlang.org/packages/video_player
Some storage plugins exist (e.g. https://pub.dartlang.org/packages/sqflite), definitely more to write here.
Always more to do. If there is something specific you believe my team should help provide, please let us know: flutter.io/support. Hope that helps!