Just as with all intro packs we've seen, you'll have to do some tinkering around with the deck list of Mortals of Myth if you plan on taking this deck away from the kitchen table and into a more competitive environment such as your local Friday Night Magic.
Ajani, Mentor of Heroes can help give Mortals of Myth some extra power, and Godsend is another amazing new card from Journey Into Nyx that'll really make your threat's fearsome. You may even consider running a god or two in the deck: Karametra, God of Harvest, Heliod, God of the Sun, and Nylea, God of the Hunt can all add late game oomph.
We realize the above cards are chase rares, and those looking to get a maximum bang for your buck are better off upgrading the deck's heroic creatures with Fabled Hero and or Anthousa, Setessan Hero in lieu of the deck's two main Pheres-Band Thunderhoof. Bow of Nylea is a versatile artifact worth considering too.