I'm narrowing down the rainbow cards in the cube. While it is possible to play 5 color, getting and supporting the colors isn't easy and it too hard to build around. Also I'm taking some of these cards for my new commander deck I've built. Ramos cares too much about color. No Ramos means no Door nor Progentitus.
Suncrusher gets back in because it doesn't need other colored spells to work. Tomb of the Spirit Dragon seems like a great life gain card. The other cards are on a trial period, likely to be replaced later.

Ghostfire is a better target for replacement for Demon Bolt than Twin Bolt, so twin is back in

I missed out on Kaldhiem so I didn't know about Foretell until I saw Delayed Blast Fireball. Placing a card into exile facedown for generic mana lets you set up spells and plays with the exile zone. This also gives eldrazi processors an interesting niche in eating these spells before they can be cast (I'm thinking of adding in hideaway for this reason too).

There is only one of each tri color card in this cube. Ideally, their goal is to help put focus on more niche archetypes with each three color trio.
Blood Tyrant didn't do this, but Maestros Ascendancy does. Grixis has some powerful spell slinging capabilities and recurring spells is a great way to spotlight that. Kess, Dissident Mage was considered too, but I wanted to reinforce the sacrifice theme as well.

Channel is too strong for the cube.
Darksteel Colossus is never chosen over the eldrazi, plus Mechtitan is the big mech on the block
Suncrusher isn't that interesting of a grab, but it will live on in the Wizard's Tower pack/ Conjurer's Closet will help with the flicker facedowns theme.
Nazahn, Revered Blacksmith and Hammer of Nazahn really play into equipment matters. I think GW's main goal is going to be to buff creatures by a variety of means, not just +1/+1 coutners.

Genesis Ultimatum needed to be replaced, too hard to cast.
Abzan charm is good, but I want the tri-color cards to be splashy and really impactful.
Conflux was left in for Wizard's tower. I recently got a new box for my cube with much more space, so I think I'll maintain a Wizard's Tower package separate from the main cube. Conflus lives there now.