DrDeth gave good advice. I would go with something that you want to play, but find a way to do that and have divine/arcane utility. You have a bard, but that isn't exactly a wizard.
If you want a melee or ranged type that heals, a Hospitaler paladin can work great. Want some more spellcasting ability, go Oracle/Paladin, the mix varies based on what you want, but consider Pal2/OracleX or Oracle4/PalX or Oracle1/PalX.
I second DrDeth's recommendations on using domain/mystery/etc for utility. Another option would be a witch. As you mentioned, but I wouldn't go so far as to take hedge witch if you don't want to. Often, locking down enemies will really lessen the damage you all take. A Scarred-witch doctor or winter witch could work well if you want something beyond vanilla. Time patron has some nice wizardesque spells - teleport/haste.
A druid is ok, but in this party I'd say no. You're really losing power as your spell slots are at such a high premium. A witch, oracle, paladin, or cleric has other things they can focus on, but you as a druid might spend a spell every fight just buffing or controlling the battlefield, etc. Go with a class that can heal that you won't LOSE effectiveness at your other primary jobs.
