For an more in depth look at my initial impressions of this card, check out this article.
Let’s start with the basics, Echo of Eons works very well with Lion’s Eye Diamond. First you play Burning Wish to retrieve Echo of Eons, then play your Lion’s Eye Diamond (this plays around artifact disruption), discard your hand and add three blue to flashback your pseudo Timetwister. Another way of getting Echo of Eons into your graveyard is to Thoughtseize yourself, while not ideal, it is a “build your own Diminishing Returns!”
Something to keep in mind is that Echo of Eons is symmetrical, which means your opponent will also draw a fresh seven card hand off of Echo of Eons. This is an issue against blue decks, to off-set this, cards such as Hope of Ghirapur, Defense Grid, and even Silence I would recommend over additional discard spells opening up another engine versus counterspell archetypes.
One of my favorite things about it is that it’s the first three mana storm engine that’s been legal in Legacy that’s playable, this helps quite a bit against Prison strategies or decks trying to lock you out of the game like Maverick or Death & Taxes.