I'd have the DCnU coexisting with the [former] mainstream DCU. Make the new universe a divergent reality, or somesuch, that occasionally interacts with the mainstream universe--much like Wildstorm used to do.
That way you can bring on new readers without pissing off long-time fans (who've grown accustomed to very specific characters, and their very specific stories) by retconning said characters entirely.
I agree with Galan. I'd also integrate the Vertigo universe more with the mainstream DCU .
For Marvel, I'd do an arc where Zeus battles with Odin (since admit it, everyone wants to see that). I'd also bring back Shuma-Gorath and Zom (but not like the lame fragment seen in WWH, I mean the real Zom). Also more Celestials.