Delver of Secrets is a notoriously fickle card to try and accommodate in cube, while most of the other suggestions require too much setup for too little payoff
Teferi, Master of Time I typically avoid planeswalker ults on lists like this because achieving them is incredibly inconsistent, but Teferi, Master of Times static ability makes it much more reliable than most and all the card filtering helps sculpt the perfect hand for those two extra turns
Its also a legit threat on its own, frequently hitting for three or more despite being a two-drop
Storm decks usually use a combination of cantrips, net positive mana rocks, untap effects, and recursion to inflate the storm count to the point where they can close out the game using closers like Aetherflux Reservoir or Tendrils of Agony
Its great at taking revenge on a specific player while also triggering draw third card effects
These do the dual purpose of both playing into the journey of our protagonist too, and acting as spells that our commander will be able to cast during combat