Does an Archetype affect summoning cost effects?
No, Archetype of Imagination does not interact with Warden of Evos Isle.

When Archetype, or any other card, refers to "creatures", it means creature permanents. So it does not affect creature cards or creature spells while they are in any other zone besides the battlefield. The relevant rule here is 109.2:


  If a spell or ability uses a description of an object that includes a card type or subtype, but doesn't include the word "card," "spell," "source," or "scheme," it means a permanent of that card type or subtype on the battlefield.