My thoughts,

On the fourth wall: I think only one person has conceived this idea from what has been written in the opening post, I myself certainly didn't consider it. Therefore I'm going to go ahead and say like any RP that does not specifically refer to a "fourth wall", there shouldn't be one, unless this RP is to be a comedy. Comedies are not my thing, so therefore I am against the notion.

On Technology: I'd vote for a typical fantasy/steampunk mixture, kind of like Warcraft, where you have your traditional medieval stuff spliced with crude and wondrous inventions. That way there's something for everyone.

On Magic: I always like it when doomsday level magic "used to be a thing", but isn't anymore. However, relics and hidden powers belonging to the distant dead are available to be found, and can become centers of interest for any aspiring villain. In summary, I'd say keep magic in the middle areas, allow moderate strength for the most gaming-minded writer, but I would advise straying away from trying to create some kind of multiplayer arena killing machine, because I don't think this RP wants to go down that road. It should be about the story, not how many other characters your character can "kill". 

On Lore and the World Around Us: Let the players make things up as they go, but include close GM supervision to prevent contradictions and outlandish craziness. Advise people with ideas they think might cause a stir to first contact the GM so that the two can figure things out. This way no one gets stuck waiting for the "person with initiative" to spawn monsters and challenges for their characters - we can all pick up the mantle, and take charge. May end up being a little chaotic if we have too many immature/thoughtless writers with us, but it would help with post blocking under the right conditions.

And that is all I have. :)

EDIT:

On the Tone and Mood: Speaking from personal preference, I like my Rps serious, although tongue in cheek comedy is welcome if executed correctly. However, I'm willing to go as far as the middle ground between seriousness and nonsense to make myself more versatile to the views of others on this subject.