Blue:
Squelch- Is this just a replacement for Stifle that can cycle when it's not relevant?
Dissipate- I hate the double blue for a 3 CMC Counterspell here. Have you tried Syncopate, which is easier to cast and can sometimes hit something on x=1?
Scatter to the Winds- feels like cancel 98% of the time. My favorite 3 CMC Counterspells in your list are Complicate, Spell Shrivel, and Exclude. I feel like they should all be single blue and either exile (or bottom) or draw a card. Maybe trim down to 2 slots instead of 3 as well?
Pestermite- I'm just very surprised that this versatile and interesting card is missing.
Waterfront Bouncer- ever played this guy?
Black:
Toxic Deluge- was this cut for Languish?
Red:
Act of Agression- have you tried this? The instant speed mode just gives some sweet options.
Green:
Curse of Predation- this was one of those hooks for going into green aggro that I thought was particularly good. Thoughts?
Boon Satyr- I just love this card. Thoughts and experiences with this? Bant Doublestrike deck? Simic Tempo? I'm always looking for solid combat tricks for cube, too.
Artifact:
Hangarback Walker- see Pia and Kiran. Too resilient? Too universal?
Gold:
Dragonlord OjutaiSagu MaulerDack's Duplicate
Xenagos the Reveler/Dragonlord Atarka- have you ever thought about making your Gruul about rampy things?
Lands: creature Lands- have you figured out how and if you want to include these?