I have the 3rd level of the new megadungeon mapped out as far as rooms and corridors go. I plan to recycle some of the old maps, using parts of them for the 4th level (I'll be redrawing them so they can interconnect better), so the next step will be matching up areas for stairways connecting the 3rd and 4th level, then adding in a bit of detail (I'm thinking that some brick-lined open sewer type waterways in certain corridors/rooms could make it a bit more interesting than the typical flagstone floors that cover most of the first two levels, for example).
I still need to do a lot of stocking on the 1st proper dungeon level, too. I think I need to start up the game with my wife in the Megadungeon after all, just so it will give me the impetus to stock that level while she's exploring around the upper works. (No, we haven't started yet. On the nights I come home early, our son's going to bed late, and on the nights I come home late, both wife and son are already asleep).