This is going to come down a lot to just what you want. 5th runes well with a balanced party but also works without. The best balance to that party would be a cleric/bard that can support/heal/spellcast.
In the end pick what you want, if you want to bend arcane magic to your will and manipulate it to benefit you farther with metamagic go sorc (or if you want to see if you kill your party or turn into a plant with wild magic). Pick a druid if you want to use the power of nature through magic or being a bear and mauling your enemies.
Its hard to really day what class someone should play, so here is quick list to pick what sounds like a character you want to play.
Bard: Raging tank on the front lines that deals damage as well as absorbs damage for the team.
Bard: A do anything class, great support and very versatile caster.
Cleric: Invoker of divine magic as a caster or a high AC melee caster, domain play a big roll here.
Druid: Harness the magic of nature to deal with enemies or just turn into a bear and maul things.
Fighter: Front lines, hit things, abilities to help with this or add a bit of magic in.
Monk: High mobility melee fighter with a number of abilities to handle difference situations that others cannot easily deal with.
Paladin: Defender of his beliefs that lead others and smite their foes, high single target damage and passive support abilties.
Ranger: Woodsman trained with hunting and stalking with various weapons to deal with his enemies, great with ranged weapons.
Rogue: Sneak around, deal with traps, deal massive damage with one attack, your stealth class.
Sorc: Handle arcane magic and modify it beyond what others can with metamagic.
Warlock: Limited spells but are more constantly restored and a number of unique abilties with patrons, pacts, and invocations
Wizard: Great knowledge of arcane magic with a focus on one school of it.