This is what Necromancer should do:
Take the 4E system, and fluff it out Necromancer style. Take races, classes, monsters and fluff them out 1E/3E style, and include the new 4E arrivals. Give the new 4E arrivals a place in the old school fluff. Expand the Points of Light with more specifics, using Necromancer's old school feel. Make it similar to a campaign setting, but focus on fluffing out all the facets of 4E available in the GSL Necromancer style as opposed to focusing on the setting itself.
Think of the fluff in the 1E/2E core books. The fluff in the complete 2E handbooks. Fluff as an interesting read for its own sake.
One thing that WotC with 4E is noticably going short on is fluff. Some people have been asking for it. If you want some low hanging fruit, Necromancer or other 3pp, this is it. Not fluff on medieval equipment or mounts or fringe things, detailed fluff on the core details of the game. I point this at Necromancer, as from what I've seen/heard this is the sort of thing I could see from them.