In-engine Doom Level Editor:

Controls based off db2/gzdb visual mode where applicable.
Grid is drawn on floors/ceilings. Grid can be doubled/halved with [/].
Freelook is necessary for aiming at floors/ceilings.
Any changes that don't require a node rebuild can be visible immediately.
Any changes that DO require a node rebuild need to be signified somehow, a preview of sorts.
The engine must support reloading a level from a wad file.
Any changes must be able to be saved without reloading the wad.
Until new UI elements can be implemented, a console will be necessary.
Hotkeys for noclip and fly are necessary
Texture selection is an issue........

+-- Don't require nodebuilding
Sector floor/height changes. Select a floor/ceiling and use the scrollwheel to change the height.
Texture offsets. Select a sidedef and arrowkeys to align.
Texture copy/paste. Ctrl-C/Ctrl-V. Should work the same for floors and ceilings.

+-- Does require nodebuilding
Any vertex movement.
New Sectors.

+-- Nodebuilding/Reloading/Saving
The wad loaded will be saved to a temporary file, possibly WADNAME_edit.wad or similar.
Any changes in the map can be saved to the temp wad at any time. This does not reload the wad in the engine.
At any point the wad can be reloaded. This will rebuild the nodes and move the player to the same position they were already in.
All changes that require a reload must still be visible, until the wad is manually reloaded. All changes will be saved before a rebuild/reload.
Reloading won't take a negligible amount of time, so the process is built around it being a distinct action the user takes. It should only take a second or two for cases outside of very complex maps (or underpowered machines).

+-- Building new sectors(cool)
To draw new sectors inside existing sectors:
- The user must aim at the grid on the floor or ceiling of a sector. A marker will highlight the closest point on the grid that the user is aiming at. An action (such as r-click) will begin drawing a line from that point. The user continues to click points until the line is closed by clicking the original point again. This is analogous to drawing sectors in db2/gzdb's 2d mode.

To draw new sectors outside of existing sectors:
- If outside of the map, the grid should extend from the nearest sector some distance. This can possibly be determined by the players distance from nearest sector.
- At this point the process is the same. The new sectors will be built at the closest existing sector's floor/ceiling height.
- The HOM effect must be disabled by drawing a solid-color background before rendering the level. A close-to-black color is preferred (#05050A or similar).

New sectors must be drawn to the screen in some way. They can be saved to the wad without having to reload in-engine but the user must be able to see the changes they have just made.
A wireframe of the sector will be drawn over the level. Once created, the sector will automatically be selected (visible with a glow or similar), and the scrollwheel will raise and lower it. In this case the visible sides will be wireframed also.
Being able to reselect a "temporary" sector like this may be non-trivial, so it would not be unreasonable to refuse the user the ability to reselect it until the map is reloaded.
The new sector will be written to the map when the map is saved, not reloaded.
Only one new sector can be drawn at a time before reloading.