Modern Horizon 3 is brimming with cubable cards!
Many uncommons are upgrades over existing cards and almost all the new designs have incredibly pushed stats.
I've been reflecting on how much I want the cube to change with any given set, as I'm trying to avoid overwhelming my new and returning players.
I'll try to reign in the complexity of cards with longer and longer textboxes.

All of the inclusions below are filling niches that I feel could use a power level boost.

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The newest iteration of Evolving Wilds/Terramorphic Expanse have the option to tap for mana on the turn they come into play. It's particularly useful to make viable playing multiple colors in aggressive strategies.
The cycling ability is very helpful to identify at a glance which colors a specific land finds. This was a problem with the Capenna fetchlands so I cut them and some additional 5-color lands to make space for all 10 Landscapes.

Denethor, Ruling Steward does not hold up to Hidden Stockpile: paying one more mana for the sacrifice ability is already a stretch, but Denethor even lacks the card selection that made the Stockpile closer to a looter in gameplay.
In his place Obstinate Gargoyle is an additional persist creature (in colors that could easily abuse that ability). There are very few creatures with persist playable in Peasant!

I've been trying to add more removal in Gruul but cards like Pit Fight or Savage Smash always felt worse than the mono-red versions of damage spells.
Stump Stomp is exactly what I wanted and it can be played as a land which seems like an appropriate effect for Gruul colors.

Leonin Relic-Warder has been pushed out by Abdel Adrian, Gorion's Ward for the same combo without needing an additional piece as an outlet. Removing artifacts or enchantments is more efficiently done by Cathar Commando and other similar creatures.
I like the option to go over the top with Angel of the Ruins, it doesn't protect itself but it immediately impacts the board and can be easily set up for reanimation.

I've been unhappy with removal in mono-green, as it is often prone to blowouts with a well timed spell in response.
With this downside in mind and a strategic incentive to "just go bigger", players regularly cut instant and sorcery removal from their green decks.
Bridgeworks Battle solves this problem by replacing a forest instead of another spell.

The time has come for Fettergeist to retire. It's individually powerful but has gradually been pushed away from blue's core archetypes: flyers, ninjas and control can't find space for such a taxing creature.
I want to use this slot to increase the count of flash creatures in the cube. Hope-Ender Coatl is the exact power level I wanted Silumgar Sorcerer to be.

In the same vein I dusted off the cobwebs on Ghostly Flicker... I can't even remember the last time I've seen it in a deck!
Hydroelectric Specimen will be played much more often, even if just as a tapped land. I believe the Specimen will generate impactful moments by diverting key spells.

I've desired to play Clone in the cube for a while but the rarity of most of its printings irked me. Copycrook is a bit harder to cast but it's immediately the best creature on the board thanks to the additional ability to grow and/or loot on attack.
To avoid the unintuitive interactions and the disproportionately match-up dependent effect of Wash Out I've taken it out of the list.