I voted Channex, Ghost Quarter, and the 1-discard trio. Channex is just too game-warping - forcing people to play land.dec or account for extra mana until it wins and gets banned is no fun. Ghost Quarter is Strip Mine here, and in a world without ABU artifact mana that's even more bonkers that it is in full range xCM. And the discard trio is too easy and versatile disruption in this format (though in 4CM Blackmail does become amusing - unlike 3CM, I can protect one card from you!). I think Despise/Duress should be the best 1-cost discard in this format.
I don't think Depths and Maniac and Shelldock are *that* bad in the Modern world (no Lotus no Led no Moxes no Petal no Ritual...). The fast mana that makes these so first-turn-decisive isn't here.
And on that note, I do have one off-the-board suggestion that would be my sixth choice of consideration for initial ban...
Simian Spirit Guide. For the most part, in the Modern cardpool all that fast nonmodern mana mentioned above is unavailable - what's in modern is generally slower or somehow gimped. The exception being SSG. I think Island-SSG-SSG-Lab Maniac is a problem deck, but I think it's the SSG, and not the Maniac, that makes it a first turn drop and thus a problem. I don't think Maniac shouldn't be a deck, I think Maniac shouldn't be a T1 drop. SSG into Manamorphose similarly makes mana happen in any color too fast.
(actually,
Chancellor of the Tangle probably has a similar concern...)
As you said in the OP: "Because mana in Modern, as compared to Vintage, is rough." - that is really the most fundamental difference here, and if the defining characteristic is intended to be "mana is rough", why let the first few rounds have those fast mana options at all?