I always do homebrew settings (with the exception of our last game, which was in Ebberon) and they are usually pretty crazy like Fallen Seraphs. I mean this in an absolutely good way, these games are awesome!
Fallen Seraph, I actually ran a game that featured a 'god machine', It was essentially a construct sun that didn't move in the sky but had massive metal plates that moved around its sphere to block out light, imitating night and day cycles throughout the (flat) earth. The concept was pretty neat, and the I love the sound of your campaign! One suggestion - The surface of the god machine can make an amazingly fun, scary, and dynamic area to explore and fight on.
Well, my players and I have been playing 'out there' games for a while now, Ebberon being one of the most 'normal' and 'classic'. So I decided when planning my upcoming game that we were going to have a return to normality. That is - everyone is a young, good aligned character, and this is a nice refreshingly classic high fantasy heroes campaign.
Yay! A break from moral ambiguity! Sighs of relief! WELL NOT FOR LONG FOOLISH PLAYERS!
Once they get their fill of "ha ha, good heroes always win! Yay love and happiness" things are going to get a hell of a lot darker, as their young heroes are pitched against the forces of madness and have to make alliances with the evil they sought to destroy! HAHAHA EVIL DM HAHAHAAAA.
No but seriously, my players love me
Good luck with your nonstandard campaign.
Edit: Syrio, I think Taint rules will be even easier to impliment in 4e. One way could be power replacement rules similar to multiclassing, replacing whatever power with another, tainted power. Just a suggestion.