Mirage block was the first block where I played Magic "Competitively" back in 1996 like many others. It was the first series of expansion sets that was built for the then new 3-set "Block" format and tested to work together independently. It has some old mechanics from the Alpha release and a few new ones like flanking and phasing.
The cube contains cards from Mirage, Visions and Weatherlight. I have added the legendary personalities from later expansions since I was thrilled to finally see who all these cards were referencing when they came out. Teferi, Mangara, Jolrael, and Kaervek.
When looking to build a new cube after the first two, I wanted something that was powerful, nostalgic and reasonably balanced. I also considered Tempest Block, Urza's Block and Ice Age "block". Mirage ended up being the most balanced with the least amount of gaps between card power, color power and overall fun.
Each color contains: 35 Creatures and 25 Spells
There are also: 28 Artifacts, 20 Multi-Colored Cards, and 12 non-basic lands
EDIT: Been drafting this on cubetutor, and it is very, very fun. Really interesting to observe the difference in relative power between spells and creatures back in the day; 3/3 fliers for CMC 5, or 4/4s with drawbacks, seemed to really have been the top tier beaters of the time!
I removed a couple Phasing specific cards that I think would be very low picks. I'm not sure Spatial Binding even works under the new Phasing rules.
Maro was a card that I wanted in from the beginning and I think Fungus Elemental was mostly just another big beater with probably the harshest drawback for the least gain among the large green beaters in the cube.
Soul Rend was too color specific. Wave of Terror gives a nice option for control and some conditional mass removal.
I played the cube with an old friend very familiar with the block last year and he had some suggestions that I never got around to implementing but I am doing so now.
There are probably too many enchantments in the cube so I am replacing some of those with non-enchantment spells and I'm also trying to increase the count of removal spells at the same time. Furthermore, defensive or slow-play spells are getting cut for more aggressive spells.
Pendrell Mists out
Pendrell Mists is too hard to play around. It's a powerful effect but the cards needed to make it effective don't exist in the cube. Pendrell Mists > Sapphire Charm
Ward of lights is another annoying enchantment (and color-hate) and Ivory Charm is more versatile with a lower cost. Ward of Lights > Ivory Charm
Superior Numbers is too conditional and Feral Instinct is a nice trick/pump effect with draw. Surprised I didn't have it in the first place. Superior Numbers > Feral Instinct
The green creature list was pretty weak considering how many big fatties that green has in the block. I should have made these changes a long time ago. The cards being removed were unimpressive and hard to use. Again, conditional abilities are not very useful.
Wave Elemental out
Wave elemental is just not good enough for it's high cost. It's power and toughness do not matter much and it's ability is replicated/surpassed by other cards in the cube. Avizoa is a more playable cost at 3U and has an interesting evasive/pump power.
Some changes for black. Kaervek's Spite is just too awful a drawback for 5 life, even as a finisher. Phyrexian Tribute is a 3 for one in cards for an artifact, and there aren't that many in the cube. Necratog is iconic, but hard to power up. Tar Pit Warrior is easier to splash and make use of. Kaervek's Spite > Ebony Charm Phyrexian Tribute > Ashen Powder Necratog > Tar Pit Warrior