The slow but steady eroding of the red lines in HS

Void Contract destroys decks, too (half of each)

Also, there are more subtle deckbuilding altering cards like Kazakusan, Kazakus, Golem Shaper, Prince Keleseth, Prince Taldaram, Prince Valanar, Drek'Thar or Vanndar Stormpike. All mage or hunter cards which work only if your deck contains spells only (Font of Power, Apexis Blast, To My Side!, Rhok'delar).

Lord Jaraxxus used to set your life to 15.

Also, Transfer Student cares for board.

Next I want the physical barrier between players removed so I can punch my opponent in the face when he pulls some RNG bullshit.

I like to think of it a bit differently and say there are basic rules established from the start of the game:

• Turn order, starting hand size and who gets the coin.
• Decks are shuffled with random card order (both before and after the mulligan).
• Draw a card and gain a mana crystal each turn. (Start with 0 mana crystals before turn 1.)
• Each turn lasts 90 seconds (plus minor modifications for turns 1 and 2, and going down to 15s if you're AFK).
• Max hand size is 10 and mana cap is also 10.
• You have a hero power that is refreshed each turn and an associated hero that starts at 30 health.
• All cards and hero powers cost a specified amount of mana.
• Card types do the following:
  • Spells are 1-time effects.
  • Weapons are hero attack modifiers on that player's turn.
  • Minions are... actually too much to bother explaining the rules for here.
  • Hero cards are glorified spells that always give you a new hero power and some armour.
    • I'm gonna be honest: half of why I wrote all these out was so I could justify calling hero cards glorified spells. I refuse to put them on a pedestal until they get something other than a battlecry/choose one/combo effect and they are no longer just complicated 1-time effects.
• A bunch of rules for keywords that no one has time to go into fully. Secrets (and maybe quests) are the only ones that really need special rules, the rest are all just shorthand.

I'm sure I missed some, but you get the point. That all defines 'Hearthstone'. Tavern Brawls, Duels and solo stuff can change some rules of course.

What's important is that nowhere does it say what card and hero power effects can and cannot do, including changing the original rules. In a sense there are no rules for card effects, there are only limitations on what the code is equipped to handle, at which point we can debate whether any red lines even exist to be eroded.

To speculate which basic rules change next, I don't actually see much that hasn't been messed with already. Even the distinction between card types has been blurred (albeit lightly) with Felsteel Executioner. Pretty much all that remains sacred so far is that weapons don't provide attack on your opponent's turn and secrets don't trigger on your own (but even that was different back in beta). I could believe they will one day print a weapon that's active on your opponent's turn, so long as it loses durability. The question is, is that actually useful enough to be worth making a card for it?

+1 to a a lot of what AngryShuckie is saying. When discussing an MTG card that seemed to violate some rules, a friend of mine quipped that the text on each MTG card basically reads "here's how I break the rules." Creatures don't get to attack the turn they're summoned, except when their text includes Haste, for example. It's easy to point to new card designs and see them as breaking rules, but a lot of times the point of a card is to do something novel, and the fundamental rules aren't violated.

I think a lot of the original red lines here aren't really red lines that the game has crossed, as they were in the original game:

• Immune existed on Ice Block, Bestial Wrath, and Gladiator's Longbow in Classic Hearthstone
• Felguard destroyed a mana crystal in Classic Hearthstone
• The existence of armor made having a health total that exceeded 30 possible for multiple classes in Classic Hearthstone
• Tech cards like Mana Wraith could impact the cost of key cards for your opponent in Classic Hearthstone. Not as strong as Loatheb by a mile, but it's still there
• There were no rules that said you couldn't restrict your deck to singleton, or evens only, or only shadow spells, etc., in Hearthstone ever - they just didn't make sense without payoffs

On a related note, there are some newer effects here that have reasonable precursors:

• Sigils are similar to Secrets
• Objectives are similar to end-of-turn triggers on minions
• Dual class cards operate in some strange middle space between neutral and class cards, and focus on shared identity of those classes (i.e. we're not seeing massive card draw added to Priest via Mage/Priest class cards - there's more commonality than that)

I think there are some real rule breaks here - like being able to gain up to 20 mana, or hold up to 12 cards, or having a card that always shows up in your opening hand - but mostly these are examples of effects which have been tuned up and down over the history of Hearthstone, not red lines that have been crossed.

I think the most egregious examples of rule breaks are breaks to constants (e.g. your starting hand is random, you only have 90 seconds to take your turn). Rule breaks on varying systems (e.g. how many cards can you have, how much mana can you have) is generally much more manageable because those are resources that are designed to change over the course of the game, so there are already cards that interact with them, making balance easier to judge. This is why Guff works (there have been lots of mana cheat tools in Druid, so Blizzard has some sense of how OP that would be and took out Maly before releasing this), and Valdris was never interesting (12 cards isn't that many more than 10, and you want to play cards, so you'll rarely get there), but Quests and especially Open the Waygate have made very problematic metas (while the OG Nozdormu was a pain for randomly generated effects).

Honestly, I don't think I'd want to see most of the other core rules of the game crossed. The balance on these things is just too hard. But I could imagine seeing more examples of Sigil-like or Objective-like spells added to the game, and maybe even secrets. One kind of wild thing I'd love to see would be a Core set with secrets for every class plus an expansion with secrets for every class released all at once at the start of a rotation. That would add enough secrets to make them work for any class, it would equalize that "I do stuff on your turn" feature that some classes have but other don't, and it would eventually rotate out so as not to become a permanent feature of the game.

I wanted to write a longer text but basically all I wanted to say was already mentioned above in a more eloquent manner. What I want to stress:  Most of the "red lines" you mentioned are in my mind not real digressions from basic game principles but a mere evolution. Imo the first real "red line" crossed was the introduction of Genn / Baku. These grant you a real big start of game advantage for a downside that is actually rather minor.

Some speculation on future red lines crossed:

• Spells that can be cast in your opponents turn
• Graveyard mechanic

The latter is probably more realistic. I could imagine them doing an undead themed expansion with some sort of graveyard mechanic

But.. they did? They were banned.

0-mana: win the game seems likely. 

Can't say I'm a big fan of these "break the rules" cards though. The biggest offenders for me have been taking mechanics that ordinarily punish you or have a certain expectation of just how much they can do, and instead, rewarding you/doing the opposite/going far beyond what is reasonable.  Ex:

• Divine Favor - ordinarily you are punished for dumping your hand (card adv vs board adv) and your opponent rewarded for using resources efficiently.  This card does the opposite, and punished your opponent for playing efficiently, while simultaneously rewarding you for sacrificing card adv.
• Demon Seed - there exists fatigue in HS for a reason.  With this card, that reason is no longer something you need to worry about, and your opponent can only 'play around it' by killing you faster, which may not be a game plan their deck even has access to.
• Jade Idol/Ignite - You're limited to a certain deck size so games aren't able to go on forever and there is a definite and predictable ending point.  Fatigue factors in to this as well, but y'know what, who cares about that when you have infinite cards in your deck that deal an infinitely scaling amount of damage? 
• Mana cheat (NOT ramp, but mana discounting) - I think playing a certain amount of value for each mana point is a reasonable expectation.  Even pushing that boundary is okay, because some cards need to be stronger.  You got a 4/5 for 2 mana?  Okay fine.  You cast 25 mana worth of spells for 0 mana?  Not fine.

edit to add

• Battlecry: <this effect is permanent>.  - need I explain?

In my opinion, maybe they just run out of ideas. In each new addon, some new card obscures the old card. For example, Icehowl and Fizzy Elemental.

Also look at those cards:

Alliance Bannerman and Vanguard Bannerman

Swolefin and Swole Squirrel

It looks like almost a literal copying from another game.

The whole "red line" premise is pretty sketchy if you ask me. From that perspective, every card with text on it is "breaking the rules" in some way by doing something other cards can't. There were never any solid (or semisolid) boundaries surrounding what a card can and cannot do.

WoTC even used to teach its card games by saying, "Here are the basic rules, but the vast majority of cards you see will offer exceptions to these rules." it's called "exception-based game design," and it's pretty normal, even outside of card games (D&D, for example).

So truthfully, nothing is off the table as long as the coders are willing and able to code it.