Install any third party app store.

Install third party apps without any connection to any Apple service, whatsoever.

Access all my user files (non-root) via MTP on my computer, without using any Apple service whatsoever.

Using it without an iCloud. Maybe possible, but, as far as I know, you have to cut a lot of possibilities, if you commit to that.

Flashing custom ROM or the ability to manage ALL apps on the phone the way I want to, without restrictions. If I want to delete an app, even if it's one from Apple, let me delete it.

Having everything work on a generic PC, for example running Linux, just as if I'd be using an Android phone.

Being able to develop on the system I use. In this hypothetical scenario I would use an iPhone, so this would be my system & I would like to develop anything I like for it, without registering & begging Apple for permission to develop something for the device I legally BOUGHT & OWN, which means, I should be able to do whatever I want with it, without any registration or connections to online services, whatsoever.