Build to Balance an All Damage Party

[Captain Panda]

Moon druid is always the right answer. Need heals? Healing spirit, best out of combat healing spell around. Need a tank? There's a form for that. Need dps? Summon a bunch of wolves or giant cobras. Need to control the battlefield? Spike growth those zombies and watch them commit suicide by druid.

 

[Mistwell]

Bard with a single level of Cleric. Be the guy who hands them dice to boost everything. Guidance die to boost ability checks. Bless dice. Inspiration dice. Hand disadvantage dice to your DM with Vicious Mockery and Faerie Fire. It will be a blast!

 

[Sorcerers Apprentice]

I think the suggested sorcerer who can twin buff spells and cast the occasional AOE damage spell to soften up big groups before the rest of the gang takes them on would give the biggest power boost to the groups in question.

You could also go with a sorlock, for more spells known and to join in on the DPR fun with quickened eldritch blasts when you feel like it.