What patch are you talking about?
For me, patch means something special; e.g. working fork of CPython where the proposed idea is implemented.

I read await coro()[key] as 
1. Function named coro() returns a dict-like object
2. An element under key is selected from dict, which is expected to be an awaitable.
3. The awaitable is awaited.

Pretty clear and unambiguous.

Changing await priority depending on the presence or absence of brackets on the right makes Python syntax very complex if possible at all.

Sorry, the proposal has zero chance to be accepted.