Source (Streets of New Capenna - "There was also a common cycle of monocolor creatures with a hybrid activation of the two ally colors called Initiates. This allowed us to make three-color commons that played into the shard theme without making cards that were hard to play. (As a normal rule, we tend to avoid making too many common traditional gold three-color cards.)")
Source (The Seer Cycle - "[...] They were moved to uncommon, and a new cycle of one-shot spells took their place at common. The common spells had the same scalable effect as their respective Seer.")
Source (Use of Multicolor - Power Spell Cycle (Rare) - "I believe this cycle turned into the Ascendancy enchantments, such as Jeskai Ascendancy ." - Empires (Rare) - "[...] We went through many iterations, but the one we went with is a cycle of enchantments that cost the wedge's colors. [...] It's possible this is the cycle that became the Ascendancy enchantments.")
Source 1 (Question #7 - "Invasion, made use of Unglued II's split cards") - Source 2 (#1) (tie) Split cards (3) - "Split cards were first created for Unglued 2, but eventually saw print in Invasion.")