How powerful would incorporeality for 7 rounds be?

[Yora]

For a kind of E8-ish game, I am considering a spell similar to gaseous form, which turns the target creature incorporeal for 1 round/level the creature would be unable to attack or cast spells with verbal components or material components, but also very difficult to injure. Turning it ethereal would also work, though there isn't really an ethereal plane in the world I am planning for.

However, the spell would also allow the target to move through walls, which might potentially be very powerful. Would that be way out of scope for a 4th level spell?

The main purpose is to make the target creature almost invulnerable but also powerless for a few rounds. Would a different approach to creating that effect be a better idea?

[rockdeworld]

Considering another 4th level spell is Polymorph, I'd say no. Making someone a scout for rounds/level would not be exceeding what 4th level spells do.

But you might not want to go by polymorph for balance purposes. Becoming incorporeal, with or without spells, is a game-changing ability because you can move through walls, ceilings, and floors, and is usually reserved for 7th-8th level spells/abilities. I've heard the point of an E6-type game is to avoid the gamechanginess of increasing levels. So if you're the DM, this might be something you don't want to allow, and if you're a player, you might want to look elsewhere for defenses.

[Deophaun]

I'd keep around some of the rules for ethereal travel, such as you can't move through a solid object thicker than you are (that limits the wall-transversing a bit).

But, at level 8, a druid can go ethereal for 8 hours a day (Aberration Wild Shape + enhance wild shape + dharculus), and a Dread Necromancer can get an incorporeal familiar (ghostly visage). Incorporeal-ness may be rare at that level, but it's not unheard of.