You either create cards that just objectively suck (i.e.: most of this list), or you risk making an actual good leyline that breaks the game by nature of how these cards work
Of course, this runs the risk of your opponents drawing that many cards and still having a main phase, but thats why you have Plagiarize, right
Like most Magic: The Gathering cards, the value of Liberator, Urza's Battlethopter depends on a few consistent factors: its rarity, the printing and finish (regular or foil), its physical condition, and the set it was printed in (The Brothers' War)
Drop that to one mana each, two total with Heartstone or Training Grounds
Return that card to the battlefield under its owner's control at the beginning of that player's next end step