Blueprints which allow to process different Editor-Only events.
Blueprints which allow to create different Editor/Viewport etc. tools / Extensions.
Also make most of C++ editor implementation behavior overridable through Blueprints. For example, make editor’s camera movement events overridable, like any other blueprint events. Also editor input/controls events etc.
The point is, to make Editor fully customizable/extendable through Blueprints.
For example, Unity3D has Editor folder which executes scripts inside that folder statically only for Editor. You could make something like Global Editor Blueprint, analogue of Level Blueprint, but only for entire Editor. Or you could make Per-Editor (Per-Widget) Blueprints, which would exist per every Editor (LevelEditor, PersonaEditor, MaterialEditor etc…, each editor could have unique set of overridable events) or even for user’s Custom Editors, like Level Blueprints exists per every Level.