The editor
I have written a few level editors, often only intended to be used with a certain project and once something that was intended to be used a common tool. Often I see it as welcome change and use the chance to work on something else.
For my current project I was conflicted about what to do with the terrain editor. I had two choices, either create a seperate tool or integrate the editor in the game. The first choice always has my preference as a tool and a game have different usage requirements which is reflected in the way they are programmed. The second choice is also a risk in my opinion as you might couple part of your codes that should not be coupled. Decoupling them afterwards is, as many programmers can tastify, a pain.
However the advantage of combining the code is that I don't need to duplicate code or functionality. This is also the reason why I have decided to create the editor in the game. The cool thing however will be I can review the results of my work in realtime.
However before I can start working on terrain editing, I need to write the terrain code, which means that I need to start thinking about the culling acceleration structure and refresh the knowledge about terrains.