I always enjoy hearing about others playing Age of Worms in 5e. This is the first AP I have DMed, and as we wrap up chapter 4 of 12 this Sunday, we are having a blast.
I would suggest not using multiclass. There is a lot of power that is foregone for multiclass, but that is just my personal feelings on the matter. So my suggestions:
1. Paladin Oath of the Ancients: The combination auras of +1 to all saving throws plus resistance to magic damage is crazy. So many times in the last few weeks an AoE (fireball, flame strike, etc.) spell goes off and becomes, "You all take 7 damage and the 1 guy that failed his saving throw takes 14, and the rogue takes 0." With the smites, the paladin is a good damage dealer, and a paladin with a shield, protection fighting style and shield master feat becomes a great ally with the auras as well. I cannot say enough good things about this class.
2. A rogue that focuses on traps and backstabbing is an effective damage dealer. Our rogue took expertise in thieves tools and perception. She had a 19 passive perception at 5th level. Pretty much nothing gets past her. It does not help that they usually put her at the back of the party... She can pretty reliably hide (even if it is behind an ally) or find another way to bring her sneak attack dice into play each round. It is also a good idea to scout in pairs to not be surprised and back each other up if the situation gets bad.
3. Story wise, a warlock of the Raven Queen would be a great choice. There are lots of undead in this campaign, and the Raven Queen has tasked you with ending the undead blight.
4. Story wise, a fighter of some renown who saw the rest of the party in the gladiator games (maybe someone on one of the other teams) wants to join the party in adventuring.
5. Story wise, a wizard who had specialized in necromancy to eradicate undead would be great, but it is not really what your party needs.
6. It seems like you would be really interested to play a Bard of Valor, a bard who saw the party fight in the arena and wants to chronicle their further adventures. Focus all of your spells on the non-obvious magic. Vicious mockery and Friends instead of Mage Hand and Light; Animal Friendship and Tasha's Hideous Laughter instead of healing spells; almost any 2nd level spell. Pretty much any spell can be wrapped up to look non magical. Tasha's Hideous Laughter and Vicious Mockery are the effective use of words, Tongues is your (intermittent) ability to speak any language, Compulsion is your compelling argument. The list goes on, and the bard can still do damage.
7. There is one damage dealing tank above any other: a half-orc barbarian (either path). Maxed out strength, dexterity and constitution for awesome damage, armor and health, doesn't necessarily go down at 0 hp, and an extra die on a critical hit. Once again, this goes well on the gladiator angle. I have a dream to play a half-orc barbarian with the tavern brawler feat and name him Banner.
