Agreed with 4 color being too greedy. Even if no one punishes you, hitting the right colors is extremely difficult for powering Nacatl early and leaving up the right combination for other things you'd want to do.

I just can't see Domri being the correct choice for a deck. 3 mana is the most significant investment you're currently paying and he doesn't add much pressure by increasing your clock or even give you significant card advantage. In terms of planeswalkers, it's one of the least scary to play against. If you need something late game to prevent stalls, I'd always go 1 mana higher for the planeswalkers that are way more powerful, Chandra, torch of defiance or Koth of the hammer or maybe Garruk relentless. You have enough card filtering to just discard them if it's early game and you don't think you can support it that game.

If you can't hit 4 mana reliably, I'd shift the creatures into giving you more reach. Grim Lavamancer is a great 1-cmc for late game even if it competes for graveyard resources, and Ghor-clan rampager can provide the extra burst you need or turn into a 4/4 creature in top deck mode. Tireless Tracker and Knight of the Reliquary are decent cards to increase your late game potential, even if you don't plan to abuse them.

Most of these higher cmc suggestions could replace Domri but as you are an aggro deck you wouldn't want to use many of them.