• The battlefield and the aetherial battlefield don't intersect. Each treats the other as though it doesn't exist.

• When casting spells that interact with permanents, players much choose which battlefield that spell is affecting. Spells with multiple targets can't target across the two battlefields, and "Destroy all..." and other blanket effects affect only one battlefield at a time.

Ah, the stygian mechanic from Theros Beyond Death, but more complicated

Would it be a Link design if it didn't overly complicate something?

I had to look up the mechanic you referenced, and oops. That is really similar 😅

Many more rules will need to be written to make this to work; so far it is not good enough, I think. There are many considerations to make.

Mmmm, also the mechanic from Raging Rivers and (Shadow) and .... there does seem to be a lot of demand for "Let's have two separate fights" among rules designers. Heck, even Shaharazad can be seen as "Put half your life into a separate battle".

Looked at that way, Shaharazad is the actual cleanest implementaiton of the concept, but suffers from being horribly slow. This is almost at the other end of that spectrum - the sub game runs entirely concurrently with it, and you have to run both concurrently with resources shared between them. (To truly go to the other end, you'd play two-handed as well; draw to both hand; mana duplicated to both fields.)

Is this the sweetspot? I think I agree with te consensus that it's not. There's a huge amount of complexity in rules and state, in exchange for, effectively, "sometimes some creatures have 'Cannot block or be blocked'".

... and that makes me think that while I said there's an end to this spectrum - the other end actually extends further; with banding being in the direction of its opposite.

On to other issues - I don't see a way to simplify even the reminder-text verison of these rules enough to allow any card to have any substantial ability being aetherial. Which is maybe OK since as long as you have STARTED aetherial, any other creature will continue it. And a one drop common to turn on this entire feature is an extremely bold statement of "Aetherial will be enabled for the next three years." Which is likely to be divisive.

One large possible pitfall - which the inability to print more than "Flying, Aetherial" scared me into - evasion abilities on top of splitting the battlefield in two is just going to be an incredibly annoying thing. Decks will have to more than double their defences against evasion. Which seems like it'll limit deck building and drafting to "Ok, I've got my 18 removal spells, which 2 creatures should I run?"

It's always my dumbest designs that end up having the most conversation. Haha

Perhaps to nobody's surprise, I didn't put that much thought into this — I just spat out an idea, partially based on flavor. I considered banking the "aetherial battlefield" the "blind eternities" since I was thinking of the upcoming story events.

Could you cast Innocent Blood on a battlefield you have no creatures on with the effect only causing others to have to sacrifice?

Yes.

Shadow is easy. That affects what can block, but doesn't make separate battlefields or anything. That's that weird part