ZOMG!
I just downloaded the latest HoMM 5 patch, and got the Map Editor, and while it will still be piecework to make a dom map out of it - boy does it look great!
At the end of the post is an editor made screenshot, it doesn't look very pretty because I just drew big sqaures of different terrain types, and the angle is Pi/2 because I was too lazy to fiddle with the camera (the map obviously looks much better in a different angle), but keep in mind that that thin strip of black stuff (the thinnest strip of color viewable) is 2 map tiles big, so you can get great detail.
Not only that, but it saves the map in .tga, so no need for conversion.
(The map is obviously resized, it was ~4200*3200 originally, it is also severely cropped, you can see about half the territory of the map)
HERE BE SCREENSHOTS (You wouldn't believe how long it took me to get this hosted. Damned Image Shack and slow Photobucket!)
EDIT:
Now this is a much prettier looking map I made specifically for you folks
Its the smallest map size available in the map editor (so I wouldn't have to work hard to make it), it is resized from 5104*3552, and showcases some of the nicer things available. There are still a few faults (mainly due to my lame screenshots positioning skills), but it is much prettier than the previous one, and shows that the HoMM 5 map editor is aa prime candidate for Dom maps in my opinion.