Here's a few suggestions...
- (Location-)hint window, which would show the hints shown in the location where the mouse pointer is. You know what I mean, a nice little window showing the hint titles.
- An option to show where there are pit gaps in the ceiling. Quite essential for designing diabolical dungeons

.
-If someone uses varying offsets, a nice transparent map of the level below/above would be great. I actually remember someone suggesting this, but I'll suggest it again. This would make my previous suggestion quite obsolete.
- Make the editor modify CONFIG.TXT self, so the lazy user doesn't have to bother tweaking that him/herself.
Ok, that's a few. If I might inquire, how complete the editor already is? Or have you chosen to wait until v1.0?