I think MMWW would be difficult for me to play. I see a setting where "an essential part of life is disappearing" to bea rather negatively oriented game. oit lends itself pretty heavily toward conflict and control, those with power clutching it ever tighter in their fists, lest they lose all, and that's just not me. I'd have to play as some sort of nonmagical character -a a pioneer of the new world rather than a refugee of the old.
For ouroboropolos - that one sounds somewhat interesting, though I feel that the setting would need to be fairly concretely established, and again, I am not sure I would be anything otehr than a sort of generic grunt there. Unless the story unfolds in a rather taoist yin yang way, in which our purging of the dark is practically mirrored by the Holy City becoming corrupt and monstrous - the white dot in the black expanding to become the white, the black dot in the white expanding to become the black. There's interesting rp opportunity there. On the other hand, I am a tad particular about things like Holiness and Evil, so I might be dissatisfied with the in game canon.
The munchkin one sounds interesting for the moldability of the characters. It reminds me of games where you start with a generic robot and custom build yourself out of scrap, found items, and lost technology. both the fantasy and the sci fi versions of this are interesting.
I don't know quite why "a game completely separate from ER" is a voting option, as PW has Infinite Heavens already, And from what I hear, he has run several others, including perplexicon, with no connection to ER anyway. In other words, this option is already a reality, the only question is "will there be another version of ER or not."
I see good an negative points for all options and have a hard time saying "I think this one is best."
Hope that helps.