Skirmish Core Prototype is a canon-version of the skeleton used for Skirmish Core.

Alright, let's put out the Skirmish Core requirements and design philosophies;

450-card Cube + 75-card Legendary Roster.
20-card packs with 3x Legends/Pack, 15x Regular Cards, and 2x Fixing Lands/Pack.
2 cards/pick
67 cards/color (335), 20 multicolor (2/pair), 10 hybrid, 30 artifacts, 50 fixing lands, 5 utility lands.
1v1 Commander (25 life, 60-card decks, no color identity/singleton rules, free mulligan, best-of-1)

The core set :skirmish: is designed to provide an easy to get into experience (lower complexity without compromising on excitement/power level) as well as provide the foundations for Skirmish.
Ordinarily sideboard cards should either have flexibility or be replaceable (cycling) to account for being maindecked.

Pioneer Power Level (Reposted from the previous Skirmish document.)
Unconditional hand attack starts at 2. Conditional ones like Duress are fine. (eg. Agonizing Remorse)
1 drop creature accelerants trending towards yes, but keeping density and removability in mind. (eg. Elvish Mystic)
2 drop accelerants trending towards yes. (eg. Diamond cycle, Rampant Growth.)
Cantrips: Keep it on the lower side of things (no ponder/preordains). (eg. Opt, Consider)
Removal: 3-mana without drawbacks, 2-mana with (eg. Infernal Grasp, Fateful Absence, Hero’s Downfall, Ban
Rituals: Nope.
Manaless interaction: Nope.
Brawl Power Staples: To be considered.

Then, there's the usual stuff like:
Designing for singleton 1v1 commander: Slightly more life means less focus on all out aggro/burn decks so growing/sticky threats over jackal pups. Extra life also means we can trend slightly higher on 2 and 3 drops.
Keep in mind commander mana values to not overload a particular mana value with cards that commander would want. (Example being Adeline on mv3 with token/token producers at mv3 vs 1 and 2.)
Avoid having too many commanders that are simply just "When you do X, draw a card."
Keep in mind density of effects for specific key pieces multiple decks will want.
"Thrivify" some nonbasic lands to ensure more decks can pick up the pieces when needed.
Focus on open ended commander archetypes vs narrow ones. Commanders that recontexualize the cube's contents are great as are commanders that are their own archetype. Open ended playstyles (control/midrange/aggro) plus broad archetypes (etbs, tokens, go wide, ins/sor spells, reanimate, ramp, etc) are also good.
Reprint canon cards when simple effects are needed (opt/consider, rampant growth, infernal grasp, lightning strike, etc.)