"The makers" have already spoken, in the form of Lukas' answer.

What makes you think Legends were meant to be untouchable? They just can't be subject to effects that specifically target units/supports/tactics, but that's nothing special, it's just like "target support" effects not working on units. It doesn't mean Legends can't be touched, or were meant to. You can deal counterstrike damage to a legend, for example, since it is dealt to an "attacker", not an "attacking unit". And there will be effects specifically targeting Legends in the upcoming cycle, as we already know: www.fantasyflightgames.com/ffg_content/warhammer_lcg/bloodquest-cycle/fragments-of-power/descendant-of-gods.png I bet there won't just be beneficial ones. Anyway, Promenade of Malice targets cards, so anything in play is fair game.

But you have to admit, that there are no clear rules on this case. For example: What will happen to a unit, if i was going to remove the quest under it? Also from a logical point of view this card seems not harmonious. How can i remove a quest from a questing unit methaphorical spoken with a building? Or how can i remove a building with a building? Is the soul of the building supposed to migrate through the promenade and is thereby cracked by an endless nightmare of terror raining all upon it?

Another question: When having watch tower out, and opponent is revealing his hand, can i trigger an ability to discard something from his hand or deck and then trigger promenade to target copycard that he has in his hand, all while he is revealing...? It's not said that the target card has to be in play, you could use it on cards in the graveyard as well, am i right?

Forget the second question, i guess the action chain is not right.

 I asked Lukas both questions (Sigmar Church and questing units), so we have to wait for his answer.

Promenade of Malice can't target cards that aren't in play. No effect can, unless it specifically says otherwise.

If a quest leaves play, the unit questing on it is removed from it but stays in the quest zone. I asked Lukas about this in anticipation of Heroic Task.

As for your thoughts on logic and harmony: This is a game where a bunch of fat small bearded dudes can demolish the blessing of the fertility goddess Isha. Promenade of Malice is not going to push it over the edge.

 @Daniello_s

Lukas answered the question of sigmar church in combination with Promenade of Malice :  "Both Church of Sigmar and Iron Discipline will affect Promenade of Malice if the card in question is a unit."