If we look at the Range section of the PHB we see this:
Spells, such as the shield spell, affect only you. These spells have a range of self. (PHB pg. 203).
Misty Step has a range of "Self" (PHB pg. 260).
Since the range of Misty Step is self it only affects you and any belongings on your person.
In conclusion - NO, as cool as it would be to do so, Misty step's range limits you to only teleporting yourself not other creatures (willing or no).
This doesn't work. To start with, you can't cast a bonus action spell and another leveled spell in the same turn.
A spell cast with a bonus action is especially swift. You
must use a bonus action on your turn to cast the spell,
provided that you haven’t already taken a bonus action
this turn. You can’t cast another spell during the same
turn, except for a cantrip with a casting time of 1 action.
Secondly, the Fog Cloud spell makes an area heavily obscured.
A heavily obscured area—such as darkness, opaque
fog, or dense foliage—blocks vision entirely. A creature
effectively suffers from the blinded condition (see
appendix A) when trying to see something in that area.
In case the blinded condition needs explaining,:
A blinded creature can’t see and automatically fails any
ability check that requires sight.
Misty Step lets you teleport to somewhere you can see - you can't see anything if you're in a Fog Cloud, so you can't use Misty Step at all.
Thirdly, it doesn't matter whether enemies can see where you teleport from. You want to hide the location you're teleporting to, so this is all pointless anyway.
Best Answer
Yes, you can misty step above someone.
There is nothing in the rules preventing this. Misty step says:
If you can see the unoccupied space, you can teleport to it. The trouble is adjudicating what happens when you fall onto another creature.
Tasha's Cauldron adds an optional rule
What I describe towards the end is how I have done it for a while now, but Tasha's Cauldron of Everything introduces an optional rule:
I cannot speak from experience concerning this rule, as I have not used it yet at the table.
In my experience.
The rules are pretty silent concerning 3d space. Prior to Tasha's, there was nothing in the core rules to help us adjudicate the spaces above the ground, so the DM needed to work with the players to determine how to rule this. This silence has led to such Q&As as this one: Does the Evard's Black Tentacles spell affect the 5-foot cubes above its area?
In my games, I have allowed exactly the scenario you describe - using misty step or dimension door to teleport directly above an enemy. In these scenarios, I give the creature on the ground DC 10 dexterity saving throw to take full or half the falling damage the falling creature takes. I use DC 10 because it makes for a pretty easy save, and discourages abuse. My players have always enjoyed doing this and have never taken to abusing it.
So for example, if you use misty step to teleport 30 feet above a creature and fall onto them, they make a DC 10 dexterity save, taking 3d6 on a failure, or half that on a success. I've always used a quick Pythagorean Theorem calculation to determine how high you can appear based on how far away from the target creature you are when you misty step. The formula is:
$$\sqrt{900-d^2}.$$
Where \$d\$ is the ground distance from the creature you are trying to fall on. Here is a quick reference table with the distances I have allowed for misty step: