I really wonder how they are going to implement the "at least 1 legendary creature in every booster pack", since historically these gimmicks have replaced a common or basic land card (see dfc's in Shadows over Innistrad and mana fixing in Fates Reforged). I did some drafts vs AI under the very basic assumption that you simply get a random legendary creature instead of a common, and I've had a few super sweet decks come together. My favorite two so far have been...
UR Wizards
Naban was awesome with the Trickster, might actually be playable given enough wizards, though it's kind of a weird tribe to cater to in cube, maybe? There's our namesake
Riptide Laboratory though, which just so happens to work together excellently with the wizards Naban wants you to run.
Keldon Overseer is a nice card for aggressive decks. Puts your opponent under pressure early, and is a great topdeck later on.
Powerstone Shard is great in multiples, no surprise there
Jhoira was sweet! There were so many ways to trigger her, and everybody knows card advantage is sweet, right!
Time of Ice was g-r-e-a-t! I glossed over the exact rules text until I played with the card, but it isn't the usual "doesn't untap during their controller's next untap step". ToI locks down all creatures it targets until it leaves play!
Karn's Temporal Sundering is suh-weet!!! Unsummon + Time Warp is a great combination if your deck can support the number of legendaries required.
Only "disappointing" card in the deck was Naru Meha, which has never copied a spell so far. 3/3 flash is still a great ambusher though, and the buff to wizards is actually relevant in this deck.
And yes, I did get to crew the Weatherlight with the captain
GU Value Train
I think I'm in love with Tatyova. Especially after I cast a Slinn Voda, and was able to attack for 4 still
(thanks Freyalise!)
Both Relic Runner and Tetsuko Umezawa provide a steady stream of pings, I like both of them as potential two drops for my cube actually. Tetsuko especially makes utility creatures relevant in the late game, as suddenly your 1/1's can get in.
Ancient Animus was good, instant speed fight effect that puts a +1/+1 counter on a legend? Yes please!
Song of Freyalise I'm still not sure about. I think I like it? Haven't seen an explosion yet though, since the two mana cost does hamper your development.
Edit: I cast
Kamahl's Druidic Vow for 6 and found
Tatyova, Benthic Druid and three lands. <3