Hedron Crawler: While this doesn't benefit from Panharmonicon, we desperately need something to early on, and it helps us cast our more expensive cards and trigger Eldrazi Displacer's ability. Plus, it's the only mana-dork in our colours :P
Glint-Nest Crane: These help to get your first Panharmonicon! And after you get your first, they definitely help you get your second and your third :)
Reflector Mage: The ultimate ETB effect in Standard. This acts like a creature AND a removal spell, and Panharmonicon only makes this card better.
Eldrazi Displacer: This card retriggers ETB effects and can save your creatures from targeted removal. Plus I really like this card and it took A LOT for me to take out the 4th copy.
Gonti, Lord of Luxury: I really really like this card, and I mean REALLY like this card (my 2nd favourite card in Kaladesh). I think the ETB is super strong, it's power and toughness is almost perfect and deathtouch is a very relevant into the later game.
Cloudblazer: Possibly the strongest ETB in the deck. If you have just one Panharmonicon, you draw FOUR cards and gain FOUR life. I mean, enough said.
Linvala, the Preserver: If you have at lest one Panharmonicon and you are losing horribly, THIS is the card you want to see. For six mana you get: a 5/5 flyer, two 3/3 flyers and gain 10 life.
Noxious Gearhulk: Where Linvala is the 6-drop you want if you're behind, this is the one you want if you're ahead or at parity. Destroy one (or hopefully more) of their blockers, gain a bit of life and get a very nice creature too. As an added bonus, you can fetch it off of Glint-Nest Crane's trigger.