Valkyrie: Splash damaging buildings too
Golem: Golemites
Witch: Spawn troops
Lava Hound: Air Def + Lava Pups

All of these features were new when we unlocked them, for example, the range of an archer was new when we upgraded our barracks, even though it seems basic now. A new troop may or may or may not be "basic" but it must be NEW.

How to make your troop new:

1. Favourite Target - eg. Lava Hound, Wall Breakers etc.
2. Special Attack/Anything while alive - eg. Archers, wizards, witches etc.
3. Death feature - eg. Golems, Lava Hounds, even Balloons to an extent.

So choose one of those 3 ways, and think of something 

Step 2 - Statistics

So now that you know what your troop is going to do, you need to think of some exact numbers that show how much damage your troop can do and take, how long it takes to train, how much it costs, and all of the other stats. I'll cover each of them one by one. Let's start with the basics.

DPS and HP

These are the most important stats, so you need to be spot on with these. They also play a big part in the balancing of the troop.

DPS (Damage per second) - You should probably decide how often your troop will attack before making the DPS. 1 second? 2 seconds? 4 seconds? Then, you look at the HP of a few buildings and decide how much damage you want each attack to do. Then you divide this by the rate of attack. Some buildings that I compare my DPS with are:

Mine/Collector
Town Hall
Clan Castle
Cannon
Mortar
Xbow
Inferno
Army Camp

Of course, you only need to see the ones that your troop will attack.

NOTE: Make sure you compare it with a level of building that your troop will be likely to attack. For example, troop unlocked at Dark Barracks Level 7 wouldn't regularly attack a Level 4 Mortar.

HP (Hitpoints) - It's basically the same concept as DPS, except this time, you compare it to the defences' DPS.

Housing Space

Imagine your troop at TH3. Too OP? Now imagine it at TH4. Still too OP? TH5? Keep doing this until your troop is not too OP. Now look at this.

TH3 - Housing Space = <10
TH4/5 - Housing Space = <15
TH6/7 - Housing Space = <20
TH8 - Housing Space = <25
TH9 - Housing Space = <30
TH10 - Housing Space = <35

You may be wondering how that works. It's because of your clan castle.

If your troop has a housing space of 16, it cannot fit in a TH3 or 4 CC. But it can fit in a TH6 one. This means that your troop will only be able to be used at TH6.

But remember, although this is something to think about while deciding housing space, you should also see how many of these troops can fit in an army camp. Example:

A TH targeting troop may not be too OP at TH3, so you might give it a housing space of let's say, 3. But then, you can fit 80 of these in a max camp, which is a lot, and a bit OP. Think about both factors.

Ground or Air?

Some questions for you:

Is it fine if the troop bypasses walls?
Is the troop OP vs air defences?
Is the troop based on anything that flies?

If the answer to any of those questions was Yes, your troop should be a flying troop.

Movement Speed

All I've got to say about this is compare your troop's speed to other troops, and use existing troop speeds to decide your troop's speed.

Training Cost

How much would YOU be prepared to pay for this troop?

How much would a farmer pay?

How much would a warrer pay?

There's no set formula to decide the troop cost, but the above questions may help you in doing so.

Training Time

Literally the same as the above ↑

Favourite Target

Think about what people would use this troop for, and if you want, make that its favourite target.

Remember, this is completely optional.

Will the troop target air troops?

Do you want your troop to be able to target:

Balloons?
Healers?
Dragons?
Minions?
Lava Hounds?
Others of its own kind (if it's an air troop)?

If you answered yes for most of them, then yes, your troop should target Ground AND Air units! Or maybe you want it to target ONLY air troops? It's completely your choice!

Upgrade Cost and Time

I just compare the other upgrade costs, and continue the pattern.

Upgraded Stats

Can't really say much here, I guess just keep some sort of pattern throughout the upgrades, and make sure it's always balanced.

Range

If you are fine with your troop attacking beyond walls, then you can have a high range. For reference:

Barbarian Range: 0.4 Tiles
Archer Range: 3.5 Tiles

User Suggestions

When creating a troop, people should think about if it would be OP in big groups. Like if the troop got to the TH in a large group of the same troop, would it 1 shot it? Try to nerf it from there. - IronGolemGaming

Step 3 - Strategies

There are 3 things to think about here.

Offensive Strategy: Do any other troops work well with this troop? Do other troops either enhance or benefit from the troop's ability? Suggest some army compositions!

Defensive Strategy: How do you protect against this troop? What base layout could you use?

Troop vs Troop Strategy: When this troop comes out of a clan castle, how can you defeat it with your troops? What troops should you use?

Step 4 - The Optional Stuff

And then there is a bunch of other stuff you can add to your troop thread if you have time, because it's fun and also shows people how much effort you put into the thread

Commercial Idea

This is a pretty fun thing to do. Make up a CoC commercial that showcases your troop!

Abbreviations and Nicknames

What will you call your troop in army composition names, or when just casually chatting about the troop?

Example:

Barbarian's nickname is Barb

Golems are Go when mentioned in attacks like GoWiPe.

What about your troop?

Detailed Appearance Descriptions

I think that's pretty straightforward.

Graphics

Maybe instead of describing the appearance of your troop, you could make some graphics for it.

Description

You know that sentence or two at the bottom of the troop stats page? You can make one.

Animations

You can think of possible animations for:

Walking
Attacking
Special Ability
Army Camp Selected

Step 5 - Making the Thread

Some tips for making the thread:

Make it look readable - Organise it, separate different sections with paragraphs, perhaps center it.

Make it look fun - Make it colourful, maybe colour code your text, add images.

Make it look professional - Proof read your thread before pressing create thread, maybe preview it, no one like readin a thrwd ful of typiws and grmmatical eerors.

Include every detail about your troop, everything you have thought of, and definitely include a credits section, to include the name of anyone who has helped you make the thread.

So have fun making your troop, and good luck in making it successful!