While nothing is stopping me from making decks with the new two-color gods as Commanders, Im already happy with the ones I have
Harmonize [cost] means You may cast this card from your graveyard by paying [cost] and tapping up to one untapped creature you control rather than paying this spells mana cost, If you cast this spell using its harmonize ability, its total cost is reduced by an amount of generic mana equal to the tapped creatures power, and If the harmonize cost was paid, exile this card instead of putting it anywhere else any time it would leave the stack. Casting a spell using its harmonize ability follows the rules for paying alternative costs in rules 601.2b and 601.2fh
Rounding things out, Id recommend Serah Farron as a great cost reducer in your Legendary-matters decks
Those 3 life points usually didn't matter much, but occasionally they make all the difference, especially when a deck finds itself in a position to do 14 or 17 damage all at once but can't do 20
The name of the game for these commanders is to control the board, benefit from graveyard synergies, and slowly dwindle your opponents decks