When it comes to Mercury, we must think of both the liquid metal mercury and the god Mercury.
The liquid metal mercury is where Merc’s color scheme of gray and silvery came from, along with it being a useful but toxic element, which ties in well with his upbringing as a highly-trained assassin. Now, the color and useful aspects of Merc is pretty straightforward, but the toxicity might have a double meaning, in that he himself has become toxic and that he had the toxicity injected into him by his father, Marcus. (Marcus is derived from Mars, which is associated with iron, red/rusted iron>>which is also toxic).
The god Mercury is Hermes’s Roman form and they are pretty much the same. Mercury/Hermes stand for MANY things, but his most well-known or important role is that he guides departed souls to the Underworld(weird connection to him being an assassin), and that he is the messenger of gods(i wonder how that part will pay out). Also he is the patron of thieves (Emerald??), travelers, and merchants, and thus is also associated with speed(godspeed), cleverness, eloquence, communication, and financial success.
The easiest Semblance to guess from all this information is speed, flight, and teleportation. But why would that kind of semblance make Merc weak? If anything they would make him far more capable as a fighter/assassin, even if it meant that Merc might be able to run from his father. Assuming that Marcus didn’t take away Merc’s semblance so he couldn’t run away (because I want to make Marcus more interesting), I’m going to assume that Marcus TRULY believed that Merc’s semblance would hinder with his growth as a assassin and took it away. For that, we need to think of a trait that assassins (or any kind of occupations that require cruelty and coldness) would think as a weakness, and I think that would be empathy (guilt and morality would be its extension).
And so my idea for Merc’s semblance is the ability to read and influence other’s emotions. I’d name it “Communication."
Communication in its truest sense is meant to better understand each other - for the better or worse - and that doesn’t simply mean understanding the spoken words; it’s about understanding their pysche, which requires one to understand people’s underlying emotions that drive them, and this also needs to be MUTUAL. So BAM, we make that Merc’s ability.
If you truly understand someone, you can never hate them. Even if that person is your enemy.
If anything if you really understand someone, you can’t help but love them.
And that is why I think Merc’s semblance is “Communication,” and that is why I think Marcus stole it from him.
And you know, if this was Merc's Semblance then I think it will be all the more interesting and relatable that he wanted to get his Semblance back. Perhaps younger Merc wanted to understand why his father hated him so much. Perhaps he wanted his father to understand him. And don't we all crave that? To understand, and to be understood? What more can you ask for with people?