The answer to your questions really depend a lot on the world. The ability to incinerate anyone you see just by looking at them isn't that strong when everyone can turn invisible. I personally just ask myself "will this create interesting situations?" if it ends a fight too soon, or presents no benefit, it shouldn't be used. I personally prefer to collab most of my character interactions so that I can take care of problems before they get reported OOC.

With that in mind, this is how I would address most of these scenarios.

[quote]
The Power of Hallucination:

This ability can be cast by a spell, magic, or other form to cause another player to see things that are not there.
Some of the questions I have about this ability include...

1: Due to this altering the way another character sees things, is this a form of power play?

2: If the ability changes the landscape, how would that change the world they interact with?

3: If the ability creates physical hallucinations, does that create concerns with Meta-Gaming and those viewing from the outside?
[/quote]

I believe that hallucinations is something that, if implemented, needs to be done very carefully. turning invisible could be considered a hallucination, as you're NOT seeing something that's there. I think it's fair because you can still be heard, so long as the person using this power cannot 1-shot a character, isn't able to turn completely invisible, can only turn invisible once a requirement is met, etc. It's really the same for every other type of hallucination. If it's too easy to abuse, it gets a nerf, if not removed outright. 

[quote]
Poison:
Inflicting poison onto another character.

1: Would this be an ability that can be viewed as overpowered, especially depending on the various strengths of a poison?

2: Should all poisons have an antidote and what would happen if you have an ability that doesn't have one?

3: Could this be viewed as Power Play as you are forcing other characters into situations?

4: How could you enforce a strong poison in a game without it becoming something that turns a character into a micro god/goddess?
[/quote]

Again, the power table of the RP plays a lot into what is considered fair and not fair for poison, curses, etc. It's not just a matter of how strong a poison is, but how easy it is to use. some poisons can be absorbed through the skin, while others are only bad if they are injected into the bloodstream or inhaled. If a poison has no cure and can kill in seconds, it's pretty much a 1-hit kill and needs to be nerfed by being difficult to administer. 

[quote]
Mind Reading:
Ability to read the thoughts of others or even view into their memories.

1: Pretty sure this is easy... Is this Metagaming?
[/quote]

In a fight, no. I always laugh when people use mind reading as an excuse to evade a blow before it comes. If you're a well practiced fighter, you're pretty much relying on instincts and do very little thinking. Once that right hook comes out, your left knee is rising without you even thinking about it. 

Outside a fight, it depends. obviously it's not very interesting if you can just pluck exactly what you want out of someone's mind like a tray of cheese and crackers. But how dangerous is the mind reader person? Could they make much use of anyone's thoughts? Is it time consuing or dangerous to enter someone else's mind. Is the person having their mind looked into aware of this somehow? 
Again, if it's interesting, I go with it.

[quote]
Mimicking:

The ability to copy another player's abilities.

1: Similar to Mind reading... Is this Metagaming?

2: What happens if the abilities have to be learnt over time? Does that mimic player learn at a different rate?

3: What happens if it breaks the power entirely?

(eg: Copies the ability to use fireballs, writes a scene with the control of fire lava, Mimic player is told that Lava control was not part of fire control)

[/quote]

The problem with this power is that it's just impossible to enforce. If I did let someone have this power, I would make sure their power was always LESS IMPRESSIVE than the person they copied, or at least the copying player could only use the power the way they saw it. ergo, a pyromancer who can create lava throws a normal fireball at the copycat. The copycat doesn't gain the pyro's entire matrix of spells, only the fireball they were almost struck with. If the copycat had a lot of other cool abilities, I would limit the number of abilities they could copy at one time. But honestly, this seems like a really easy way to make a character OP if you're not careful.