My first attempt at a small cube. There's two ways to build each color-pair and a few worthwile splashes for some archetypes.

WU: Blink & Heroic
WB: Extort & Warriors
WR: Equipment & Tokens/Combat tricks
WG: Convoke/Tokens & +1/+1 counters
UB: Control & Reanimator
UR: Spell-slinging & Treasure(/artifacts)
UG: Self-Mill & Turbolands
BR: Madness & Sacrifice
BG: Graveyard value & Recycling
RG: Power 4+ matters & Ramp

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Key cards and strategies

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WU Blink

Blink and bounce your own creatures to reuse etb-effects and to trigger their own or other creatures' ltb-effects. Win the game by making your opponent's removal irrelevant and by accumulating value over time.

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WU Heroic

Cast Auras and combat tricks on your heroic creatures to trigger their abilities and to protect them from removal. Win the game by out-tempoing the opponent.

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WB Extort

Hide behind big butts and cast cheap spells to make the most of Extort. Win by slowly draining your opponent's life force.

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WB Warriors

Pick the cards that say Warrior on them...go wide and win.

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WR Equipment

Accumulate Voltron by putting Equipment on creatures that know how to wield them. Alternatively, use double-strikers. Win the game by hitting hard and fast.

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WR Tokens

Put as many creatures on the battlefield as you possibly can, then use mass pump to push through enough damage to win.

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WG Convoke

Make use of your wide board to cast bigger spells, to grow your creatures, and to tap down your opponent's. If necessary, use mass pump to win even more.

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WG +1/+1 counters

There's lots of +1/+1 counters in there and several cards that benefit from those counters. You'll figure it out.

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UB Control

Put up your shields and prepare for the long game. Use (mass) removal and counterspells to disrupt your opponent and eventually win with a big finisher.

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UB Reanimate

Put cards from your library into your graveyard until you hit a haymaker. Or discard one, if you have it in your hand. Reanimate your big finisher and hold up some sort of protection just in case. Win.

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UR Spell-slinging

Draft as many noncreature spells as you can afford to make the most of your triggers. Spells with flashback give you additional triggers and spells that make creature tokens give you some board presence. Sling spells until the opponent is no more.

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UR Artifacts/Treasure

Accumulate treasure tokens (as well as other artifacts) and either use them to cast bigger (or off-color) spells, or to throw them at your opponent's face until you win.

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UG Self-Mill

Shovel your library into your graveyard and use flashback spells and similar effects to increase your options. Use spells that reshuffle cards into your library before you mill yourself out.

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UG Turbolands

Use ramp to get to eight or more lands quickly in order to turn on your creatures and to cast big finishers.

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BR Madness

Pick discard outlets (ideally cheap/free ones) and cast your Madness spells with a discount and at instant speed to catch your opponent off-guard.

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BR Sacrifice

Put some creatures on the board and kill them to satiate your bloodlust. At a certain point, the opponent is doomed no matter what they do because every creature of yours that dies means damage for them.

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BG Recursion value

Put as many creature cards into your graveyard as you can to power up your spells. Return the value creatures to your hand and grind the opponent down.

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BG Recycling

Make use of every single card in your graveyard to take control of the gameflow. Win by demonstrating proper resource management.

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RG Power 4+ matters

Get out some early high-powered creatures to trigger your cards' effects. Use said big creatures to deal big damage to your opponent.

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RG Ramp

Those who control the mana, control the universe. Ramp out big creatures and activate expensive abilities to win the game.