I think Metalhawk is the most necessary at the moment. The guy's been front and center in TDOOP and RID, but we haven't really gotten inside his head much. Arcee's next on my list, as #4 has hinted at fleshing her out beyond her original Furman characterization, and that'd be great to see continue. A "day in the life" issue, perhaps centered around someone like Tappet or Zetca or that green tank guy would be cool as well.
Honestly, I'd LOVE to see pretty much anyone on that list, though.
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