The 3 life gain creatures + felidar sovereign would let you nicely round out your mana, maybe add more dorks to support the Jeskai Ascendancy plan
Really, having great energy-generating creatures like Whirler Virtuoso or Roil Cartographer in your starting hand along with three Plains feels like cruel punishment from the Magic gods
In the casual Commander variant, you can use an oversize card as your commander - a commander starts the game face up on the table and tends to stay that way, so as above it doesn't hurt that it's oversized
Personally, I would be happy to have Mulldrifter, Shriekmaw, Nevermaker, and Slithermuse at the ready, maybe even fresh off evoking
As an answer, Perilous Vault is perfect
They really showcase the power and the might of the spirit dragons, and it's just a beautiful cycle of cards by Dominic Mayer