Man, one thing is for sure - you have good roster and multiple good choices. You probably will get many suggestions here and they all will be viable.
Serris is a beast, maybe one of your better choice here. but you already have AoE Decrease Defense and Weaken, so if you want some extra CC in dungeons... may be. But it wouldn't be my personal choice at that moment.
Sinesha can be either good damage dealer or off healer support, but you have good flash heal and speed buff from Apo and you have strong damage from Draco and Kael (poison and raw damage).
Lightsworn offers some good utilities and can be good choice, but i doubt he can boost your overall progress much. He will help, sure, but can't do it by large margin.
Coldheart - you need some serious babysitting before you can rely on her. You will get her to 6/60 for sure at some point, but now she will give you not much of a boost by herself.
Yaga, Warmaiden, RelickKeeper, DoomScreech - pretend they don't exist. They are either mediocre, too situational or you have better substitute of them.
Coffin smasher - good addition to your CB team and FK dungeon, cheap to build, but since you want someone that will add to your overall progress, he can't be your default choice just now. If CB is your high priority area, then you can consider him, but.. i don't know, not enough overall value.
Maulie - will bring valuable utilities for dungeons and Arena. Good choice, but maybe not the best here.
Skullcrusher - it my seem strange i will not suggest him as CA champion right away, but i think you have better option at this stage. Still, he is your second best choice i think. You can't go wrong with him, but i think there is one champ here, who is slightly better than him in terms of most overall progress value.
My personal choice here would be MM. You already have AoE Decrease Defense and Weaken, Speed buff and Heal support, very good damage output (poison and pure damage) and MM will be that one guy that will carry you through the Dungeon content and NM campaign like a Boss! Maybe he can be viable option in your Arena too, for extra protection and CC, so the other can survive longer and deal with the enemy. But in dungeons he will bring so much value and you will see significant progress there. And higher stage Dungeons will give you better equipment to boost your Arena and CB performance until you start to work specifically for that content.
If that was my account and overall progress is my current goal, he will be my default choice.