While more variation in the game would be interesting, and another potion type or two would be cool, I once again see a few problems with it.
Potions are already a rare enough resource, and with a Grim or two not everyone even gets to actually use them. With the current three, if you actually use them and don’t carry a Grimoire, you still are able to find at least one of them useful, depending on your career and equipment. Not every one, though. With some careers a Concentration potion is near-useless (in my opinion). With another potion type, finding these useful potions, or at least the one you want, gets harder. One-in-three to one-in four is a big change, especially on top of finding potions in the first place.
Another general problem is the eternal one: Balance. Whatever the effect, it needs to be on par with the other potions, and not break the game in any situation, no matter how rare. For this reason alone, I think if there will be one, it will not be in the near future, but after we’ve gotten the balance working in existing places in the game.
For the specific ideas… A defensive potion would be useful, sure, but I’d prefer a slightly more indirect approach than straight damage or hit resistance. Infinite Stamina, maybe. Cleave on most weapons is no problem, and while the Speed’s effect on it is indirect at best, I think the Strength might actually affect your Power, which would increase cleave too.
Immunity to Specials, no thanks. You can already avoid any disabler by dodging, and a Gutter runner can be, with good timing, pushed too. Skills needed, but I don’t think a Potion (or other mechanics, for that matter) should be substitute for them.
Etherealness would be interesting indeed, but ten seconds is far too long. Half that, maybe. A bigger problem is, it would mess up Concoction. And if by “mystery potion” you mean one with random effects… No, not outside of Deeds at least. (Although it would indeed be an interesting Deed condition, but not enough by itself.) If it’s not reliable, it won’t get used. Even if the effects were just the “normal” potion effects, I would far prefer to know what I’m getting. And frankly, Ranald has pushed his finger deep enough in this game already.