Single-player mode would be a clear DMCA violation. Not even DMCA, just copyright law in general. It would require duplicating and distributing content that the players never had locally -- mission scripts, any number of serverside xp or reward tables, NPC dialog, and so forth.
To amplify, the problem is that City of Heroes didn't have a single player mode, things like Icon notwithstanding. The actual *gameplay* only existed as a conjunction of the game client and the game servers. Without the servers, there is no real gameplay: no NPCs, no powers, no missions, no combat. To get a "single player City of Heroes" you'd actually have to create something, not just "turn on" something: you'd have to create your own servers. Even if those servers were designed for single player play, and only ran on a single player's computer, and didn't allow anyone else to connect to them, the act of creating them would be a copyright violation.
If the City of Heroes game client actually contained all of that content, and had the ability to actually run missions and AI the NPCs around and execute combat and it was just turned off normally, you could maybe argue that turning it on would be a single player City. But that doesn't exist in any form whatsoever in the game client. Both Icon and Paragon Chat just enable the game client to do things it could always do, without adding any content the game client doesn't already have.
