By that dichotomy, we are all casual here. Every last one of us. Even the most hardcore player is not going to do many raids in City of Heroes. Why? We have one raid. One. His name is Hamidon, and we know the exact way we are allowed to defeat him. We know this to be the only way because all other options were nerfed within a week of being successful. Our one raid requires at least 60 people to have much chance of success. As such, no guild can monopolize it or even do it alone. Also, everyone who shows up gets a reward, so there is no reason to exclude anyone who will not get us all killed.
We have a few things to do at max level:
1. PvP. You know this one. Few people use the arenas, although more are with the new gladiator content (use NPC mobs as your puppets to fight).
2. Keep playing. Does that character have every single badge in the game? Could you use another million influence or prestige? Do you just like playing that character and feel like continuing? After all, playing at level 50 is just like playing at level 49, except you are not moving the xp bar.
3. Powerlevel friends. This happens less often than you might think, especially given how CoH/V is designed. You might as well make a new character and level it along with your friends, because your high-level powerleveling opportunities are not much better than normal play.
4. Make an alt. This is where CoH really shines when compared to many other games. It is probably the most alt-friendly game out there. Why?
a. Sidekicking. Your level 4 character can be scaled up to play with your level 38 friends. Granted, you have fewer powers with weaker enhancements, but you are still a useful teammate.
b. Different classes. Your re-roll interest increases if your characters play materially differently. Each of the five archetypes (per game) has various powersets that play differently. To me, most Blasters and Scrappers are the same, but their secondary sets make them play differently (say, Regeneration vs. Super Reflexes). In my favorite archetype, Defenders, each plays radically differently. Some people still do not grasp what exactly Dark Miasma and Kinetics do. If you can run through the same content from a different perspective, it can feel like a very different game.
Although only so many times.
c. Global chat. If you are going to encourage players to make alts, set up a system to let them keep in contact with friends without a 500-member friends list. City of Heroes lets you set an account as a global friend (subject to friend approval) and find them on another character, another server, or on the other side of the Hero/Villain divide. There are out-of-game tools to do this elsewhere, but if you are a casual player, why should you need them?
If you want your players to run through your content over and over again, make a system that encourages it. Do not make me abandon all my friends, either because I am the wrong level or because I just cannot find them buried on some alt.