Use Case
imagine multiple conditions (Change, KeyPress, …) that lead to the same actions

What are the actual use cases envisaged for this? In the abstract they would seem to either be equivalent to or substantially overlap with the existing apex.actions API:

The actions interface manages a collection of actions.action objects. An action encapsulates the identity, state and behavior of a named operation or procedure that the user initiates via a user interface element. Actions are most useful when an operation can be initiated in multiple ways such as with a button or toolbar button, menu, or keyboard shortcut. The operation should be labeled consistently and if it can be enabled and disabled that state must be kept consistent. By using an action and then associating a button and/or menu item with that action all aspects of the action are centralized and kept in sync.

Consequently the best way to achieve this would be via the long-anticipated unification of Dynamic Actions and actions.

As such requirements can be easily be implemented at present using separate DAs or actions that use apex.event.trigger to fire a DA on a custom event I don't see this as a high priority.

well for me it would be good to have that Option in the GUI: imagine you want to react both to a CHANGE and KEY RELEASE Event leading to the same Action.

Thats my personal Use-Case.

imagine you want to react both to a CHANGE and KEY RELEASE Event leading to the same Action

I would use the existing Input event in that case.