On mill cards and decks:
There are mill cards. Say, cards that make you or the foe discard the top cards of their decks or make them draw faster: Embezzle, Innovation, Chaos Seed 2.0 (and Pandemonium 3.0, the upgraded version) and Burn Card.
Some time ago, Innovation didn't make you discard the top card of your deck ('Mill 1'), just one card of your hand, and it was considered OP. It let me build insanely fast and consistent Demigod OTKs. Now it makes you discard the top card of your deck and it's... almost unused. It was a heavy nerf.
There were some decks that depended on milling the foe that depended in Ricochet and Innovation/Burn, which have been nerfed: Ricochet targets you 50% of the time instead of mostly the foe (Rico + innovation targeted a random card of both hands, and your deck was prepared to have few cards in the hand so it statistically targeted the foe much more). They were way too good, and now they are average at most.
Pandemonium 3.0 is practically only used in Arena because the AI, who can predict the RNG, makes the foe discard A LOT so there are some RNG Discard decks of Pande 3.0.
The other used combo that mills the foe is Black Cat + Embezzle + something that kills the cat (plaque, immolation, etc.). It hasn't been proved too good in PVP yet to be nerfed, but it's not bad if you get the foe unprepared.
Discarding the top card of your deck (or a random card of your deck) is risky and it would need a heavy advantage to be good.
Making the foe discard too many cards WOULD be OP (Voodoo costs just 2 quanta, imagine making the foe easily discard 16-20 cards), but the Black Cat + Embezzle combo proves it's not broken yet.
On your card ideas:
Cerberus: just no. a 2-card combo making the foe discard too much would be brrroken, and we have Voodoo and Armagio. Same for attack, we have Shadow and Ball of Lightning that would make the foe discard too much. Embezzle is expensive already, which prevents that from happening. I sincerely have never seen a PVP deck based on Embezzle + Shadow, and I'm tempted to test that.
Make a thread per card idea so we don't discuss both of them in one thread!
Bicorn: