If you had a choice, what kinds of things would you want the long and common magic discussions to figure out? There are so many of them, and so many people thinking and speculating over the same things again and again. It seems like during the time it takes to reach the magic arc, many near-perfect theories about it's possible structure could be laid out for you cleanly and clearly. But do you want that?
We're not going to use any one system, so I think the most useful things are probably general and possibly unusual ideas about how a given magic system might work in a world, where the discussion is aware that the system isn't exclusively used by all worlds, or discussions about differences that could exist between worlds. Specifics can be useful if they stay above the numerics/algorithms for the most part. I'm sure there are useful exceptions, so I don't want to exclude anything in particular. Questions of overall atmosphere are somewhat settled (as in the DF talk quotes from before), though I still like to hear opinions on all sides there. In general we're not as receptive to explicitly leveled firebolts or steampunk as we are to something more nebulous and less technological, but we don't want to shut down potential useful avenues of discussion.
Ideas where different manifestations of magic have underlying structural connections with the world are good. We're probably going to be adding different metaphysical systems along the lines of how a creation story using a pantheon might generate pre-world-gen events, and it'd answer all the big questions like "what happens when you die?", "what happens when you dream?", "why do we exist?", "is there a purpose to existence?", "what are emotions?" etc. etc., and gods, planes of existence, magical systems and whatever else can provide answers to these questions that further manifest themselves as part of a magic system (or the world could be utterly mundane as desired). When we do this, we'll be in a position where we are using our own ideas and continuing to look through suggestions and just using the best and easiest stuff at first, and that'll be how magic manifests itself.
When the economy/site resources go in, are we going to see more logical equipment for guards, invaders, and the like? Also, how well is the game going to handle in modded-in materials and the like?
You mean stuff they could reasonably afford or some other bug? Or you mean equipment based on more AI and siege planning? I'm not sure what angle you meant by more logical. For materials, it depends on how they are modded in. The current materials are all in the raws, and presumably anything people put in will be used in the same way. It has trouble following arbitrary reactions now if there are materials hidden down a production chain, but stuff in the ground and smelted metals should be handled whether it is stock or modded.
Will Nightcreature dens have theyr own crude (weapon)-traps at some point?
Probably still waiting on kobold and goblin (and dwarf) traps for things like that. And the larger human burial structures. I'm not sure if I'm going to add any adv mode traps before the mechanics rewrite, and there has to be some sort of mitigation, since the traps as they stand would tend to be fairly deadly instant game-enders for the poor explorers, although I'm not sure how that'll change.