When I was new I filled up all available slots then started swapping those out for better characters.
Now that I've got a maxed out team and I'm auto farming Trooper Badges for Skill Shards I just replace the least active trooper with new Eddie requests since most characters can't auto farm madness because of misconfigured talismans and/or missing skill shards.
Then I hit all the Secret Locations and auto farm Underworld Madness (5 action points and better rewards than Dungeons) or the Dungeons VII (7 points, the min for best rewards) if I want that color shard.