First of all, my common mantra about the land base: "If it always enters tapped, it needs to have a significant effect". Especially
Gateway Plaza: I don't think its worth paying 2 mana for colorfixing in Gx.
Then I would reccomend to replace 2 lands with
Cultivate and
Collective Voyage. The former helps you mana- and colorfixing, the latter could give you alot of land drops later on.
Goblin Bombardment is a very good card for Omnath. With your commander on the board, it allows you to sacrifice your
elemental tokens for 4 damage each. With enough elementals (
Collective voyage says hi), you could even shotgun the whole table. You could cut
Tooth and Claw for the slot.
Since you're producing some more tokens,
Impact Tremors could be interesting.
Rhythm of the Wild would be a great (and flexible) haste enabler.
Ramunap Excavator could come handy for some additonal land drops in combination with your fetch lands.
Besides that, are you sure that you don't need removal?
Nature's Claim,
Krosan Grip and
Seal of Primordium would be a good start on a budget.
I've just checked one of my decks for some more token production:
Second Harvest and (if you got the money)
Parallel Lives.