Phew. I've got a little folder of character ideas I want to play, but 3rd level with no plans to level up really narrows things down. What's something that's got a clearly defined starting subclass feature that would be fun to play around with?
One idea I've been toying with is the GOO Warlock illusionist. The main draw here is that your enchantment and illusion Warlock spells all ignore V/S components. If you're clever, and your DM doesn't shut down illusion shenanigans out of hand, there's room to get funky with that. Take Mask of Many Faces and Misty Visions for at-will Disguise Self and Silent Image, grab Minor Illusion as well, and adopt the character quirk of "constantly fidgets with their arcane focus". You'll be able to throw illusions left and right with no one the wiser. And you can always fall back on Eldritch Blast spam if it comes to a direct fight.
If you want to playtest the new toys, you could do worse than a Berserker Barbarian. Try out Weapon Mastery, improved Rage, the brand new Primal Knowledge, and the completely redone Berserker subclass. Maybe roll them as a goliath from one of the new types, to really fill out the new material checklist. At the core, it's still a Barbarian's Barbarian. But that's probably fine for a one-shot, and it's nice to see the new toys in action.