City of Heroes is a very old game and unlike games like Warframe, World of Warcraft, and Final Fantasy XIV, it does not have much in the way of linear content to push you forward with the sense of 'constantly progressing'. It's instead a game that goes in a variety of directions, but not always forward. It's a true sandbox MMO where you generally have to make your own fun while waiting for updates. I highly recommend playing the Dev's Choice AEs if you get truly desperate for new content, as they're usually pretty good.
The fun of City of Heroes is how many ways you can play it, so if you're someone who wants to play just one character, there's nothing wrong with that, but you might find more opportunities in alts and it'll feel like a completely different game.