Since you have not used your level 11 character sheet yet you're essentially "level 10" =P
By multiclass I'm assuming from your previous posts you wanted to multiclass to a fighter for the action surge to be a once per day Ion Cannon XD So since you have 2 levels you could grab 2 levels in Fighter class for:
- Fighting Style - Increasing your AC +1 by picking Defense, or Dueling for the +2 hit (since you have advantage from Reckless Attack I'd prefer Def over this); Protection is if you're constantly sticking to your teammate which isn't how Barian fights so we can scrap that Style).
- Second Wind - More HP per short rest!
- Action Surge - Unleash your Spirit Bomb once per day!
If you go Barbarian 12 you get:
- Relentless Rage - It's decent because it's situational I suppose; sometimes it's the table turner other times it's just survive one more hit before dropping.
- ASI - +2 Str or Resilience +1 Dex (adds your AC as well as your Dex Saves)
- +2 Maximum HP from Barbarian as opposed to Fighter levels
- Also have 2d12 hit dice instead of 2d10 hit dice (from Fighter)
There's no "right answer" here, just more of what you want to do~