I've got inspired to make this cube by @BenjaminWheeler's One Drops cube which (at least on camera) looks like it packs a much bigger punch than anybody would expect from a cube consisting entirely of 1 cmc spells. However because of my hubris I wanted to go even bigger and dare I say better, and what's better than 1 if not 2? I love gold cards which are basically nonexistent at 1 cmc therefore I had to double that and go up to 2 which is just perfect. Therefore my restriction for this cube is that every card and every part of a card (split cards, adventures, etc.) in this cube have a cost of no more than 2. Because of the way I set that limitation Fire // Ice is a valid card for this cube even though technically it's a 4 cmc card but Giant Killer crosses the line because its adventure costs 3 mana.
My goal for this cube is to cultivate an environment where "cheap" cards feel much bigger and the impact of casting 2 drops feels similar to casting the biggest spells in the game. This goal is also supported by inclusion of ultimately "fair" cards that allow players to play a full game of Magic instead of just spectating how your opponent steamrolls you. Because of that cards that feel unbeatable (like Mother of Runes) or easily snowball out of control (like Pack Rat) got cut after the first draft.