Major changes log:
Changed all cards that referenced coin flips to the correct Magic phrasing.
Rebalanced spells with Flashback
Added "this spell", "Add C", and "any target" wording from the Dominaria update to all appropriate cards.
Added a small milling theme in the Water archetype, replacing some of the discard effects which didn't really fit in Blue.
Added the Item subtype to non-equipment, non-mana rock artifacts so it could be referenced by other cards and already had a cohesive mechanical theme (two activated abilities like Mirrodin's Spellbombs)
Changed the OHKO spells to all Instants and rebalanced them. Fissure, Sheer Cold, Horn Drill, and Guillotine.