Key to the City would help you cast your colossus and improvise cheaper, tap for filtering, card draw, help cast
metallic rebuke and
reverse engineer, help get your big creatures through.
Lifecrafter's Bestiary seems good for draw and filtering especially with all of your Mana dorks and it does make your Colossus and your improvise cards cheaper.
Rhonas's monument looks good, especially in tandem with lifecrafter's beastiary- make your late game mana dorks basically a diet
giant growth, also 2-4 cost artifacts can really make your colossus or improvise stuff nearly free. I also figure
untethered express can grow to be really ridiculous.