I have been looking around the new Blender 2.8 interface now that it is approaching a finished state, and I've noticed that in order to to use the 2D Animation context, you have to actually create a new file specifically in that context. Further, it doesn't seem to be possible to switch from the 2D Animation context to the Modeling, Sculpting, UV Editing, etc. Should I take that to mean that if I wanted to create an animation that blends 3D and 2D--say, a 3D scene populated by 2D characters--that the best way to do that is to create a "General" file and just deal with not having the 2D animation tools quite so easily at-hand?
If you click the + icon at the end of the row of available workspaces, you will get a drop-down menu with all the other workspaces not currently in the menu.
There’s no reason why the 2D workspace couldn’t be added alongside the others, they’re just preset window layouts. You can create your own 2D animation layout and customize it to suit your own preferences.
Another option is to add a grease pencil object in the 3D workspace in object mode.
Go to Add => Grease Pencil => Blank. Then you can switch to draw mode under the ↹ Tab menu. From there you can draw into object surfaces or just aligned with your camera view.
You can select surface and set a low offset so it's not occupying the same space as the face you're drawing on.
This is a great option for drawing details that you can then convert to curves and from there a mesh that you can solidify. It can also be used for stylized texture painting.