Karn, the Great Creator - Does "from outside the game" have to be Standard if we're playing casual?
This really depends on what you mean by casual.  Karn has a card specific ruling:


  In a casual game, a card you choose from outside the game comes from your personal collection. In a tournament event, a card you choose from outside the game must come from your sideboard. You may look at your sideboard at any time. 


But if you are playing a specific format, the banned card list for that format prevents the use of those cards in general.  In tournament play, it is sufficient to ban them from main decks and sideboards.  In casual, this should be extended to include cards pulled in from outside the game.

See Spike, Tournament Grinder as the exception that proves the rule.  Spike is a joke card (not legal in any tournament formats) that lets you use cards that are banned in the current format.  Normally you cannot do that when pulling in cards from outside the game.