Where submit a feature suggestion for Android ?

Depends on the feature, it might be a AOSP push request, or a OS feature request

I guess Settings App -> System -> Feedback. But i think this is an OS feature request (like Jet in the other comment said), which is being handled by the manufacturer of the device you're using instead of the actual AOSP. I'm not sure where you can leave feedback for AOSP, if that's possible at all. Hope someone else can add that here.

The process for submitting feature requests is here: https://source.android.com/docs/setup/contribute/report-bugs

Also, apps on modern Android are mostly sandboxed so they wouldn't be able to "block" your phone on their own unfortunately. You could have one that You make/install that could do "things" but you'd be really limited without root access to your phone.

As for the phone locking when you plug it, you can make an app that detects when the phone is being charged. What I don't remember is if you can force-lock your phone from user space Android-provided APIs. Keeping it on while plugged is possible by checking the appropriate checkbox in the developer settings "keep screen on when plugged".

One feature I'd love to see, is the phone immediately locking when you plug in power, requiring you to unlock it. And then having the option to stay permanently unlocked with said power.

This would be useful when you're using the phone at a desktop, you just want to keep it open.

Immediately locking prevents somebody from plugging in a power bank to keep your phone from locking if you forget it or they snatch it out of your hands

Well I'm riffing, I want trap apps. Like a banking app, downloaded from the Play store, but if you open it it shuts the phone off completely. Again good for mugging situations