MicrosoftDavid_Edinburgh To-Do already has the ability to be used with multiple accounts (albeit not on the same screen), and it does have lists (groups?) and steps for tasks you create in To-Do, so it's come a long way from being a single-root task list. It's only when items are synced from other services like OneNote or Planner, they go into a single location.
I don't actually understand what you mean with "it would be ideal if OneNote structure is created to map roles, programmes, projects, etc. and a similar structure can be mapped in my Next Action list in MS To-Do." Maybe I'm missing out on OneNote features, but I personally like the simplicity of To-Do and for more complex task management I use Planner. Which, in turn, lets me manage all tasks assigned to me in either place.
