A full Magic experience in a box! This format was created by Brian DeMars, and is incredibly customizable to your own preferences. Brian has updated his project into a multi-functional set of cards. That list is here:
https://cubecobra.com/cube/list/dangerroomv2

The last published version of the full Danger Room is here:
https://cubecobra.com/cube/list/dangerroom

Check out another excellent list here!
https://cubecobra.com/cube/list/czbb

The power level of this last list was increased a bit, and the card pool is expanded to:
105 for each color
30 for each two color pair
6 of each three color wedge/shard
70 colorless cards
1 Fusion Elemental
1 Niv-Mizzet, Supreme (provisional basis)

Change considerations:

Footlight Fiend
Raging Swordtooth
Siegehorn Ceratops

Design Philosophies

• Mana is a fixed resource. Each player will start with 10 lands in the command zone; one of each basic, and five dual lands that come into play tapped. Because of this restriction, a player can make a maximum of 10 total mana, and no more than 3 of one color on any turn. Any cards that make mana, create Treasures, or affect land in any way are not included.

• 4 card starting hand, 7 card hand limit, start at 25 life. The previous 5 card starting hand, 9 card hand limit, 20 life was from Brian's design. Brian did have a low to the ground design that allowed for early plays more often, so depending on average mana value, these parameters could be tweaked. I have opted for a slightly higher power level, and found that 25 life gives the correct amount of tension.

• Separate libraries, separate graveyards. This is just a preference, and there is no reason that players couldn't share a library, a graveyard, or both. I've largely taken out mass card draw, which is lopsided in 1v1 play, and opted for more card selection, with looting/rummaging effects and scry/surveil effects. Card advantage exists, but is generally costly or slow, to keep games close.

• Morph is gone. Brian focused a lot of his stack on morph cards to allow for more playing to the board, rather than holding back spells. Morph is fine, but I've found that the guessing game strongly favors those with many matches using this list. Another ding against morph is explaining to newer players that morph flips don't use the stack. Ultimately, I decided that I could use the spirit of the morphs by including a foretell package instead. Foretell allows for the hidden card with cost reduction, but the full spell is generally fine as well. Often, morph creatures are way below rate if played face-up, and have no enters-the-battlefield abilities. In the end, I did keep one of my favorite cards in the list as an homage (Ethereal Ambush), and with Duskmourn, the superior manifest dread mechanic is on a handful of very interesting cards.

• Regeneration is gone. Another holdover from the olden days of Magic, regeneration is just confusing to explain to newer players. The templating of "indestructible until end of turn" is much cleaner and easier to grok for most players.

Design Deep Dives

Mutate
One of the key differences between the Danger Room and my list is the inclusion of a Mutate subtheme. When this mechanic was released, I was keen to try it out and see how it worked. I think that the power level is probably correct for this list, and the biggest mark against it is the confusion of how mutate works, as it's somehow more complicated than Bestow. Ultimately, it played well, but any time a player got two mutate creatures in one game, Voltron was very strong. I ultimately trimmed some of the creatures so that mutate exists, but sparingly. I believe the correct number is around 15 for this size box, which gives a < 5% chance of a player seeing two mutate creatures in a game where a player sees 25 cards.

Because Mutate in multiples was really good and I was actively avoiding it taking over a game, my design assumes that a player would see no more than one mutate creature in a given game, and therefore the creature has to:

1. be good enough on its own,
2. have a reasonable cost/effect assuming that it would only mutate once, and
3. not warp the game around it due to high power level or conflicting design philosophies.

These odds were calculated on an 956 card list.

Assuming a longer game (each player sees 40 cards):
Number of Mutate creatures Chance of drawing 1 Chance of drawing 2+
20 37.6% 20.2%
15 34.9% 12.7%
10 28.7% 6.2%

For a shorter game (each player sees 25 cards):
Number of Mutate creatures Chance of drawing 1 Chance of drawing 2+
20 32.1% 9.4%
15 27.4% 5.6%
10 20.8% 2.6%

Here is where I stand on the mutate creatures.
In

Could be in

Not special alone

Too powerful

Too weak or not exciting

Straight exclusions