Hearthpwn

Shell (must have cards you can build variants around):

• Baleful Banker x2
• Zola the Gorgon
• Reckless Diretroll x2
• Doomguard x2
• Soulwarden x2
• Bloodreaver Gul'dan

You no longer 'must' discard the first Soulwarden to get infinite Soulwardens and other cards.  Doing it that way is still good, but this shell lets you do it another couple of ways. 

Discard Baleful Banker (easier to do early in this shell) to Reckless Diretroll, and then as early as 8 mana you can put the Soulwarden that brings the Baleful Banker to your hand back in the deck, and then repeat when you draw it again!  Putting it in the deck instead of your hand also gives you more time to address hand management problems that are more common with the regular loop.

The third way is to discard Zola the Gorgon, which, at 9 mana, you can play Soulwarden then Zola a copy right back to your hand.  As with the original method, because you get it right back in hand, hand management issues are accelerated.

Failing to activate 'infinite' recursion by discarding one of those three cards, you can still get a LOT of non-infinite loops.  Either a natural or looped Augmented Elekk will allow you to play a natural Baleful Banker on a Soulwarden to put two copies back in the deck (four total if you manage it with both), and Zola the Gorgon gives another in hand. So even without the infinite loop, you can play Soulwarden up to seven times!

Yet another advantage of Baleful Banker is that if you discard BOTH of them, you can still discard any two other cards, and be guaranteed to preserve the infinite loop, because you will ALWAYS get at least one Baleful Banker back out of a Soulwarden's four card bank, as long as you have slots open in your hand for all three cards that return! 

Similarly, if you over-discard by more than four cards, having two Baleful Bankers (and/or Zola the Gorgon) in the Soulwarden's bank makes preserving the loop much more likely.  Your odds of getting non-loop cards back remains impacted, but as long as you can keep looping, you are likely to get any non-loop win conditions back eventually.

[Work in Progress; check replay showcases for more until later!]

TLDR Mulligan Guide Up Front:

Always Keep:

• 1 Reckless Diretroll (only) AND 1 Baleful Banker vs all EXCEPT Druid
• Skulking Geist and Demonic Project vs, All Druids
• Skulking Geist vs Hunter & Shaman
• Demonic Project vs. Deathrattle Hunter, Druid, Shaman

Situational Keeps (only after the above must keeps are in hand):

• 1 Stonehill Defender or Tar Creeper
• 1 Defile (especially in some match ups like Paladin, Shaman, and Warlock)
• 1 Hellfire (especially vs. Hunter)
• High Priestess Jeklik (against aggro, if you already have 1 cheaper taunt)
• 1 Soulwarden only with the nut draw, and never vs. Druid
• Zilliax with The Coin vs. Hunter, Shaman, Warlock
• Doomguard vs. Odd Rogue and Deathrattle Hunter, otherwise only if you are gambling hard
• Voodoo Doll vs. Warrior
• Bloodreaver Gul'dan usually NOT, unless you have the nut draw 
• 1 Seaforium Bomber or  Elise the Trailblazer, ONLY with The Coin AND Augmented Elekk (this is a gamble to stick the Augmented Elekk on turn 3)

Must Pitch:

• Lord Godfrey

The two card nut draw (except against Druid) is Reckless Diretroll and Baleful Banker, with no other two drop (Defile or Demonic Project).  Add The Coin as a three card nut draw to 'activate' the Baleful Banker loop on turn 2. 

A second best version, that you don't try for so much, but recognize as such, is to have Zola the Gorgon in place of the Baleful Banker. 

See the last paragraph of the Shell Improvement spoiler above for why discarding two of either of these cards early is really good.

Druid: Your BEST match up.  Keep Demonic Project or mulligan everything except Skulking Geist trying to get Demonic Project, because you have a better chance of drawing into board clear (if its Token Druid) than to draw Demonic Project. 

You don't need your combo at all against Malygos, Hakkar, the Soulflayer, Mecha'thun or King Togwaggle... you just need a well timed Demonic Project.  I hard mulligan for those and haven't lost to Druid yet.  Skulking Geist will cripple their end game combo, too.  It's also helpful against Hakkar, the Soulflayer builds...if they have two win conditions, you need both answers.

Hunter: Unfortunately against nearly all versions of Hunter (ever a tough match up), you will want to wait until you have played the Skulking Geist before you can safely expect any Bombs to stick, or until they have played both Trackings, or you can lose Bombs to more than just overdraw.  So even though it is slow, Skulking Geist is a likely keep. 

Otherwise, against most everything else, if you get a Baleful Banker and a Reckless Diretroll, definitely keep them. This deck frame and the accompanying package are built to maximize your odds of discarding the Baleful Banker or Zola the Gorgon (less reliably) to the Reckless Diretroll early, which sets up the new loop. 

You do have to play around Defile and Demonic Project getting in the way in your hand sometimes, but both are essential to the list, so, sometimes you just have to deal with it and hold off on discarding unless you must, until you can play those out.  Or you can gamble if you feel like you must... sometimes you must.

(New 1/29) Same game plan and success as with Mecha'thun Druid, where Skulking Geist and Demonic Project are your win conditions. 

I didn't wait to play Skulking Geist because I wanted to get rid of the 1 cost spells FAST.  But to deal with late Corrupted Bloods, you may want to Baleful Banker or Zola the Gorgon your Skulking Geist as soon as played.  You see in this replay how fast he dealt with it once played, so maybe I should have waited. 

If you don't get Skulking Geist or Demonic Project (unikely) before Hakkar is in play, you will need to stay at exactly 9 cards in hand (See Kripp's video on the thousands of points of damage Hakkar can do if you don't stay at exactly nine cards when he is in play) to avoid their combo until you can either cast Demonic Project to deal with King Togwaggle, or play Skulking Geist, or defeat them:

https://hsreplay.net/replay/u9HLSx4aFyEjZLJh4VcKJ

Long game, had to survive wave's and win out with a delayed loaded Soulwarden, banking Seaforium Bomber and two Augmented Elekks.  Got an assist when I could not enable the infinite part of the loop (failed to discard a Baleful Banker, Zola the Gorgon, or Soulwarden) by using the Augmented Elekks to one time Baleful Banker key extra copies of Soulwarden and Bombs into the decks.

https://hsreplay.net/replay/JZv3swWRKbaPZQ5DX5dWre

T12 Win controlling a double Hench-Clan Thug opening from Budget Odd Rogue!: 

https://hsreplay.net/replay/kEfwnX9RY89scs5pqmnqiE

Two unlooped Soulwardens into two Bloodreaver Gul'dans for T12 value win vs. Deathrattle Hunter 

• T5 Doomguard pitches Bloodreaver Gul'dan & Hellfire,
• T6 Soulwarden for both,
• T8 Crack egg & Clear Devilsaur trio,
• T10 First Bloodreaver Gul'dan played,
• T11 2nd Soulwarden for 2nd Bloodreaver Gul'dan+Hellfire,
• T12 Bloodreaver Gul'dan played for lethal:  

https://hsreplay.net/replay/qM5NnjiDQW7AYfyWc2X6eU

Hail Mary T8 Demonic Project into T9 Lord Jaraxxus vs. Spell Hunter for the T13 win at 1 health from T9 27-1 hole!: 

https://hsreplay.net/replay/jfcfA2Pn6wj7BZukU5M5P4

T2 activated Baleful Banker Loop vs. Spell Hunter for T9 win: 

https://hsreplay.net/replay/KjCtFEeHc9d8BnpbcN5UkV

(New 1/30) No loops, poor curve, but Skulking Geist kept in mull did major work for a T10 win!: 

https://hsreplay.net/replay/UtLVRzaKG6mrLnqnVBcBi7

Seems unfavored (no wins to share yet).

Win w/Three Bloodreaver Gul'dans off T2 activated Baleful Banker Loop vs. Odd Control Warrior!: 

https://hsreplay.net/replay/mUkKdaaxZBVF2A7y9Ee6FS

Opened with the two card nut draw (Reckless Diretroll and Baleful Banker).  Doomguarded Zola the Gorgon and Voodoo Doll into Soulwarden's card bank!  That's a killer start against any Control Warrior.  Had to hold three discard minions the rest of the game, but outvalued the tar out him. playing Soulwarden five times (sixth in hand) along with a few Seaforium Bombers and Voodoo Dolls along the way (not to mention all the Baleful Bankers and Zola the Gorgons)!

https://hsreplay.net/replay/MgHYPunphvkkePqkZWSMUN

Loss vs. Elemental Big Hand Mage: 

https://hsreplay.net/replay/gtTFmG78SPnryDE8cocGwd

Win w/Board Control Vs. Zoo Warlock: 

https://hsreplay.net/replay/h5kEsYDcQXFPzbNXaqv2BB

(New 1/29) Shatterred w/ Bombs from 4 Seaforium Bombers all Augmented Elekk'd, just before he could win!: 

https://hsreplay.net/replay/imuXXn5vjVzF9Qou7Gexo8

Timing Mecha'thun Druid's assignment to a Demonic Project!: 

https://hsreplay.net/replay/9MgtJZS4ZVT7jBzCq8qrX8

(New 1/30) Close one, but judiciously passing on draws just for the sake of using the mana allowed the win with 1 life left.  https://hsreplay.net/replay/pfyPCFugcE3TRW7GaHz4xS

(New 1/29) https://hsreplay.net/replay/YxuP89qF6yVjKUPgzsynZj

Played Topsy Turvy Priest thinking OTK Mind Blast Priest (tried to impact Shadow Visions discover options): 

https://hsreplay.net/replay/rb8p4vcTSnr4bPwvnh6Chb

(New 1/29) The replay that got away showed up! Thought the server issues on 1/29 lost it, but it was just delayed.  Played a Zerek's Cloning Gallery quest priest Hakkar, the Soulflayer variant built around Spirit of the Dead and Dead Ringer, which let him get infinite cards.  He got that activated, as I did my infinite loop, and after 25 turns, I was able to cast Bloodreaver Gul'dan for the FOURTH time (T10, T13, T21, T25), flooding the board with charging Doomguards ftw.  Epic!  Note, I deliberately did NOT always kill the Spirit of the Dead when I could, because I was trying to keep his deck full enough to avoid his win condition, even though at the time, I didn't know what it was, lol.

https://hsreplay.net/replay/3rY43HujcTf9Yd4nBRLqRR

This will be a work in progress, with more to come if I get to 10 up votes.