What are Rule 0 decks? Essentially, they are decks that break the normal rules of the game and require permission from the table to allow them.
Some examples of Rule 0 possibilities include using banned cards and silver-bordered or acorn stamped cards, or adding a mechanic that does not appear on the card (but probably should). One might think that this only applies to casual games, but they would be wrong. There are "no banned list" events for various competitive formats, and they are increasing in frequency. Another thing to consider is that EDREC shows a small number of submitted decks with cards that cannot be played without Rule 0 help. Yes, this does happen "in the wild."
So, should you build a Rule 0 Commander deck? It's a deckbuilding question that is somewhat uncommon, but getting more frequent as new cards often beg the question.
Cards That Already "Break" the Rules
Do you recall a time when Relentless Rats was limited to a one-of in Commander? The original idea of EDH was that it was, in fact, Highlander. However, once Commander's very fundamental limitation of one copy per card was removed, the floodgates continued to open ever wider. Furthermore, the format has reached a point where there are a massive number of functionally equivalent cards, so it's a bit disingenuous to say it's truly a Highlander format anymore. This is where a Rule 0 deck can give you a little bit more flavor to work with if you're interested in expanding your deckbuilding horizons.
Contrary to what Mark Rosewater says, people are regularly making Rule 0 decks. This has been true of my experiences every week at casual Commander, both on SpellTable and since 1994. The one unifying principle I can ascribe to Magic players is, paradoxically, diversity. We all collect, trade, and play this game for many different reasons. Deckbuilding is equally diverse.
Some Cards That Need a Small Push
Hedron Alignment is a perfect alternate win condition card, except it is functionally impossible in Commander. Rule 0 is the fix! Allow four copies, and instead of stifling player creativity, watch as one or two players try and incorporate the card into an existing archetype. This is not going to make your games worse, less fun, or less interactive. If anything it's going to add another element to fight over, or allow the game to develop a little bit more before concluding.
Remember Caw-Blade? Letting the spirit of this deck relive its glory days in the Commander format is completely within bounds. Is any table, from casual to competitive, really threatened by three additional 1/1 Birds? Let loose the hawks of war and see that it adds to the history and flavor of a deck without offensively upping its power.
Undead Servant is another story entirely, as it works in a completely different way and is four mana. If you allow an unlimited amount of copies in a deck, it's still easily countered via graveyard hate, and only accelerates well with many, many copies. I think that some, but not all, Zombie decks would love to have maybe ten or so copies, but none would go all in. The only way to find out would be to show up with a Rule 0 deck and see!
There are plenty of cards with the same template of getting multiple copies, and under normal rules, they are functionally useless. However, with a little bit of creativity and the permission of the table, you can be free to experiment with these cards. It's called "the Gathering," right? Keep in mind that future themes will develop, so it helps to have an open mind when new deckbuilding opportunities arise.
Just Some Broken Things
In terms of competitive viability, there are powerful mechanics to build around. Players already use Thrummingstone with Rats and Petitioners. There are plenty of cards that combo here, like Cathar's Crusade or Impact Tremors.
For a much more abusive combo, Cadaverous Bloom allows you to turn your creature search spell into extremely large quantities of mana. Broken? Well, that ends up being a five-cost enchantment with another three-cost creature spell to get massive, but not unlimited, mana. Eight mana to not necessarily end the game on the spot is far from out of the question in casual games. Plus, think about the openers in a deck that has 30 copies of Battalion Foot Soldier. Can you keep two in your opening seven? That doesn't sound very unfair.
Alternatively, using Lost Legacy on your own creature can result in a thinned deck and a pile of card draw, a potential wincon alongside another card like Thassa's Oracle or Laboratory Maniac. Clearly these are additional ways to win, but nowhere near the absolute most efficient or competitive, especially using the cards mentioned. Furthermore, that card and others like it are counters to your own strategy. Imagine someone playing Meddling Mage, Nevermore or Declaration of Naught against you.
Finally there's Zombie Infestation, which I think is one of the most broken interactions if you powered up Gathering Throng or Battalion Foot Solidier with a no limit Rule 0. This means that you could T2 Infestation, T3 search 50 cards and put 25 2/2 Zombies without haste into play. Maybe you kill one player, maybe. However, this relies on everything going your way, and there are so many commonly-played cards like Propaganda and any four-mana board wipe that stop you from winning the game. To me, it feels like a classic degenerate deck, which I think are fun to see once but not regularly. The fact is, you do not need Rule 0 help to build degenerate decks, so there is little point.
In or Out? Mostly Out
Boon of the Wish-Giver-type cards have always been a little bit controversial in Commander, and for good reason. The idea is that within your 100 card deck, you really ought to have plenty of room for a variety of cards. However, we've blown away that limitation, and demonstrated you have access to far more than 100 cards in modern Commander. That means that cards that reference even more cards from outside the game don't make a whole lot of sense in the format. Beyond being format flavor fails, functional Boon of the Wish-Giver effects are simply too powerful.
That's why I bring up both Booster Tutor and Ring of Ma'rûf as counter-examples. You need a vastly high degree of randomness and/or a massive cost, in Ring's case ten mana and a skipped draw, to make these cards merely interesting and not game-ending. Most of the Boon of the Wish-Giver effects at two or three mana, however, are simply too good to allow. To Rule 0 them in a high powered, no-banned-list-level environment makes sense, but not for lower-powered games where more fun cards like Booster Tutor can spice up a game without ruining it.
Flavor With Some Power
This is an example of a Rule 0 "partner with" combination that is played at one of my local stores. There's nothing extra powerful about this pairing, in fact, Edgar Markov's Coffin // Edgar Markov's Coffin is more powerful. However, because of the thematic nature of a set featuring Vampires getting married, you know, literal partners, maybe they could have had the partner with mechanic? They made Wedding Ring and Rhoda, Geist Avenger and Timin, Youthful Geist, but somehow missed an obvious flavor slam-dunk. Well, one local Commander player stepped up to the plate and succeeded where Wizards failed. This pairing has never been vetoed because it's not overpowered and makes too much sense. I also suggested to them that they include three copies of Wedding Invitation, one for each other player.
Furthermore, many Planeswalkers are suitable as commanders even if they lack rules text saying so. We know this because Oathbreaker is now a recognized format! So yeah, the community is better at this than Wizards, and Wizards knows it. Obviously Oathbreaker is not a one-for-one analogue for Commander, but it is pretty darn close. If parts of the Magic community can come to this conclusion without express company endorsement, can't we all?
Not a Waste of Time; a Time Investment
So should you build a Commander deck that utilizes Rule 0? Absolutely. Unquestionably. Maybe? I'm joking; of course you should! Deckbuilding is a skill like any other, and building under even loose limitations increases your experience. Sometimes, breaking a limitation shows you why it is in place and even how you can work around it. There is always value to perspective, even a completely different one from the norm. So do yourself a favor, not only for fun and flavor's sake, but also to increase your flexibility, awareness, and adaptability to future printings and developments
Have you ever built a deck using Rule 0 in Commander? Would you allow one at your table? I'd love to hear your thoughts.