I know I read somewhere that because Sleep and Overrun were both in Magic2010, Development forced Negate into common so there was at least one good answer. I can understand why a person would want Sleep variants at rare. "I play a bunch of creatures and then you lose" gets old when it happens too often.

Strangely, I can't find any cards that would just give all your creatures flying UEOT. There's Magnetic Flux, but that is just garbage.

­Levitation says that such an effect could certainly cost less than 4 mana.

Mmm, this is a "break the groundstall" card, which is a good thing to have in a set, but it should cost four or five, and be uncommon.

Unblockable is a bit of a harsh thing, though. Maybe flying?

Still better than Part Water. ;)

You are seriously overvaluing this effect. On 1vs1, Sleep gets this done and more for a lot less. There's a lot of cards you can compare this to, but they all say it's overcosted and of too high rarity. Archetype of Imagination, Ruthless Invasion, Panic Attack, Ghostform, Open into Wonder, etc...

Also, "unblockable" isn't a thing in MTG anymore.

updated to use treasure tokens

In a vacuum yes, it is worse than Start your Engines but as battle Cry will be part of the set/block, it will often be better than it.

Battle Cry is going to be an element of the set so it will show up on regular creatures as well

With two verhicles this is still worse than Start your Engines since it costs more and pumps vehicles for 1 less.

Or two vehicles and a bunch of small pilots. But it still seems like it will be hard to make it worth it.