I actually read through five and half pages of this thread before I started skimming, so I apologize if I say something that was already said.
First, the idea of seperating Dungeon/Poly or killing Poly is not going to work. Neither magazine was working on it's own, even under WotC - after all, they didn't say "axe Poly", they said "axe either". Obviously neither one of them was viable from a revenue standpoint. Seperate them, and you'll see both of them perish in a short time, IMO.
I think that there's a lot of potential in both Dungeon and Poly which isn't being utilized. I don't want a magazine that's just adventures - that's too narrow a scope for a magazine, especially one of this nature. I think they should take a look at the subscription and sales rates, and decide how to divvy up pages that way. If the subscription rates were (made up) 9k Dungeon and 1k Poly, then I say definitely cut back Poly - make it a 90/10 split. But without hard numbers, I really couldn't say what a fair split would be.
I actually think some of the articles from Dragon about world-building and DMing should go in Dungeon - especially if it's thought of as a DM's magazine. I think there needs to be a lot more generic stuff - the idea about outlining a town was great, and I think there needs to be more along those same lines, from both Poly and Dungeon.
On the Dungeon side, I think that setting-specific stuff needs to be toned down within the adventure, but perhaps expanded in a sidebar. For instance, you have a generic pirate adventure - now how can you tailor it to Greyhawk or Forgotten Realms? That way, you please people looking for any of generic, GH, or FR stuff. I also think there definitely needs to be more of an emphasis on how you can take the core books and use them in innovative and creative ways. Dragon is for new stuff. Dungeon should be for getting the most out of the core books in your adventures.
As for Poly - there needs to be first and foremost support for the major d20 settings that WotC puts out. I would say at least an even split between Star Wars, D20 Modern, and new material. I would favor something more along the lines of 35/45/20 respectively, but that's just me. Again it depends on sales statistics, though I wouldn't devote any more than 25% of Poly to new stuff. Mini-games are fine, and can be interesting if they're good. Keep supporting the good ones, screw the bad ones. One thing I definitely wouldn't mind seeing is how you might be able to use non-RPG sources to enrich your games. For example, there's all sorts of information on how solar systems might look - positions of planets around various types of stars and such. Why not include an article like that, that will be useful to people playing in space settings? It wouldn't have to be long, and could work as a good filler in slow months, I think.