1) ... Mellored posted a Vhuman Monk Open 10 with Alert , dex + dex ; for a pvp comp on wotc right before they poofed. He explained his tactics, and no one bothered to take him on.
I countered that specific build (=metagaming) with a worked up Dex Barb 7 / early UA Swash 3 just to get nearly equal on initiative+surprise ; hoping for enough hp to handle the stuns that did get through, and after the stunning is done, go with Adv and better damage. Fight never happened.
That Monk build works from level 5+, and has a nasty trick at 11, especially for arena 1v1 fights. I wouldn't bet on the Barb+Swash against that Monk.
2) ... the UA NPC article has an Alt Rolling detail that makes any Basic Rules npc considerably better. It's technically Random class first, and only srd, And No Feats = sad panda. Doing this with Frenzy Barb that blows frenzy right away, is possibly
very strong ; it's also good for the cleric or wiz or base thief. Not sure how well a kitted cleric or wiz is with your setting.
3) ... Volos has the few prebuilds. Don't know if you wanna go that restrictive on the various classes. A rogue Thief 13 might be a very sneaky way to (re-)Introduce higher level magic into the game. Surprise surprise but otherwise vanilla rogue.
4) ... Sorc 3 / Lock 3 could get the Disguise all the time + Disguise kit going , with the Focus replacement Hat or Shard in XGtE ... from the stands
, running Vicious Mockery. Fighter feels bad and wants to know why? Nobody knows. King says fighter loses and the opponent hasn't even shown up yet.