It would be cool, if we could add sub tasks to a single entry. The parent should be expandable, so that sub tasks would be visible at a glance, and progress should be visible without expanding the parent task.

the ability to invite others people in lists or activities

Outlook Mail
Flagged items appear as Tasks
Integrate or replicate functionality of Outlook Tasks where you can assign start date, due date, status, priority.. etc

Calendar
Items or meetings on my calendar should showup in a list, this helps in planning the day

People
Items like Call <someone on Contact list> or Follow up with <someone on contact list> would be nice if they are connected and you can just click on the persons name name and get details (email, phone, calendar, status..etc )

Cortana Reminders
Any reminders added in cortana should show up in To-Do

Add location based ToDO reminder list..

Attach word doc I am working on so I can open directly from todo entry and continue.

Tags would allow organization across lists and folders.

Say I have a few tasks that all have the tag #amazon, I could hit the tag and see what other things I need to grab from Amazon regardless of the list or location.

Another useful example would be for modules in a project, #backendAuth #frontendAuth #databaseStoreProcs, regardless of which sprint/list they're located in.

It would be nice to have dark theme.

I'd like tasks from both my Work (Exchange) and personal Microsoft accounts to be accessed from To-Do

You might have Most Important Task, or Important Task, or even a Bump List. Some Tasks are more important than others, So a Priority Level would be nice.

Level 1 to Level 10

Or Task Catagories: "Simple Quick Tasks", "Bump List",

Make Microsoft To-Do the one stop solution for tasks and reminders in Microsoft Products.

The app should give a list of current todos on the back side of a live tile

A widget on iOS devices which shows your today's lists.

Microsoft Flow currently supports Wunderlist and Todoist. Offering support for Microsoft To-Do would mean, that a lot of automations could work with this service as well (e.g. create a To-Do if I'm mentioned in a certain mail, etc.) This would enhance this service greatly

"My day" panel is really useful, but we also need look and have present always our week goals. The possibility bump all our to-dos help to declutter our mind, but this doesn't have sense if we can't organize or order them. So by this reason think is necesary a week view or a monthly view, even a more especific dayly view (maybe by hour or by morning/afternoon/night) could be really productive.

Brett McKay talk about this topic in this video
https://www.youtube.com/watch?v=KNxLNY6yxRI

Please allow 2 way sync with OneNote ToDo tag.

Please add reminder feature, reminders could be set to go off for a given ToDo. Integration with Cortana and Outlook Calendar events/reminders.

Import from google keep.