I like both versions - unit twists as well as permanent battlefield (or this might perhaps even include permanent global) enchantments for the different planes. At some points, these two can even meet: for example on the plane of Air, every single unit could have a Guardian Wind enchantment (which will of course be removed from them as soon as they leave the plane of Air and they will regain it as soon as they return - as long as they stay).

The plane of Fire could actively burn (damage) every single unit on it each turn by a certain amount unless they are natives (and with this, I don't mean units of non-Fire-planar factions trained on the plane of Fire, but any original, true natives of it) or protected somehow. (Come to think of it, continuous damage per turn for non-natives might also be considered for the Negative plane...)

The Negative plane could have a permanent Eternal Night and/or Evil Omens effect on it.

And so on...