I have not run out of content on alts so far. They’ve been leveling up faster due to WQ access in addition to campaign stuff. My second 70 didn’t even make it all the way through the second zone before hitting cap.
There are a ridiculous amount of side quests, so I doubt you have done them all if you have completed the main campaign and WQ and are only at 68. The main campaign alone will take you to level 67.
I also have to say, What good does it do to give alts the “option” to level as they wish when they really are pigeon holed into, “do the campaign again.” Seems like a fake choice.
While leveling? Gosh no. In fact, I hit max before I finish the campaign, and ALL I do is the campaign. I’m on a 70 now and I’ve got so many quests I don’t know where to begin. Clean out the quest log? OMG, every quest leads to another one. Such a MMO problem.