I don't want BB to just turn into "let's play through an entire game because we want to." We already kinda got away from the original intent of tackling one specific super hard achievement or challenge with Demon's Souls. I felt okay about that one because arguably the entire game is the hard part, but turning the whole thing into a glorified series of Let's Plays just feels cheap.
That weird hidden final level in 3D World that everyone says is so balls-out impossible would be perfect for this, but the legwork required to even unlock that thing would take more free time than I have right now.
If you're looking for achievements to take on in Breaking Brad:
Duke Nukem Forever - "Balls of Steel" - earn 1,000,000 points in the pinball minigame in Duke Nukem Forever
Rock Band 2 - "The Bladder of Steel Award" - perfect for the Big Live Live Show
Halo 3: ODST - "Vidmaster Challenge: Endure" - in Firefight, on any mission, passed the 4th Set on 4-player Heroic Live co-op
Left 4 Dead - "Untouchables" - no Survivors take damage after contacting the rescue vehicle
Left 4 Dead 2 - "The Real Deal" - Single Player - Survive a campaign on Expert skill with Realism mode enabled
Halo: Combat Evolved Anniversary - "Speed Reader" - Single Player - complete the level "The Library" on Legendary difficulty in 30 minutes or less.
Call of Duty: World at War - "The Sum of All Zeros" - Single Player - down 45 Japanese Zeros in "Black Cats"
Hexic HD - "Grand Pearl Pooh-Bah" - surround one piece with six black pearls
Hexic HD - "Big Cheese of the South Seas" - Form a cluster of three black pearls.
Mirror's Edge - "Superstar" - Single Player - Attain a Time Trial star rating of 90.
Call of Duty 2 - "Veteran of D-Day" - Finished D-Day mission on Veteran difficulty.
Dorito's Crash Course - "Sore wa kantan desu" - Single Player - Finished Japan level 4 with a time of 1:40 or better
Trials Evolution: take your pick
Half-Life 2: Episode 2 - "Little Rocket Man" - Send the garden gnome into space.
Geometry Wars: Retro Evolved 2 - "Smile" - Single Player - 2, 4, 11, 15, 17, 18, and 19
EDIT: I tried to pick stuff that shouldn't take too much set-up, doesn't require you to play through an entire game on a hard difficulty, are difficult achievements, and should be fairly entertaining to watch.