I’m building a completely casual deck with friends, and the the restrictions on Ring of Ma'rûf aren’t entirely clear. Could I, for example, use Ring of Maruf to gradually bring in a YuGiOh card such as Exodia The Forbidden One. Could the Ring allow that?
No, you can only get a Magic: the Gathering card. The meaning of the word "card" is given explicitly in the rules:
108.2. When a rule or text on a card refers to a “card,” it means only a Magic card or an object represented by a Magic card.
Moreover, you can only get traditional Magic cards, so no Archenemy schemes, Planechase planes, etc:
108.2a Most Magic games use only traditional Magic cards, which measure approximately 2.5 inches (6.3 cm) by 3.5 inches (8.8 cm). Certain formats also use nontraditional Magic cards, oversized cards that may have different backs.
108.5. Nontraditional Magic cards can’t start the game in any zone other than the command zone (see rule 408). If an effect would bring a nontraditional Magic card into the game from outside the game, it doesn’t; that card remains outside the game.
So while you could choose your copy of All in Good Time with Ring of Ma'rûf, it would not be able to enter the game due to 108.5, and you effectively just waste your draw and exile the Ring for nothing.