Not all of us can handle or want a Dark Souls experience when playing City of Heroes! Many of us love the game just the way it is now. Still if you must change things, here are 3 possible ways to introduce more difficulty without affecting the game as it stands:
Create some additional missions with higher level difficulties. Some of the Incarnate Trials are pretty challenging and require tight League organization to complete. A few more mid-level and Incarnate-level Trials would go a long way to satisfying the people who want more of a challenge.
Add some new Radio style missions that are a higher challenge, but don't require a League to run - for teams of up to 8 players. Maybe don't call them "Radio" missions, but some new category all of their own. Possibly something like the redo-able missions in Dark Astoria, except designed for a wider range of character levels.
Create new Villain Groups to battle in enhanced, more challenging missions/arcs.
Summary:
Please please do not increase the difficulty of City of Heroes! Instead, add more challenging content for those who want that level of difficulty.