Here's the homebrew Artificer archetype I have created. It is a subclass for the artificer class from Unearthed Arcana: Artificer.
Note: The link contains the most recent iteration. The archetype displayed in this thread is kept to avoid invalidating previous answers.
Basically, the short answer for why I made this subclass/archetype is because I like the idea of having a mech war machine.
This started as a sort of support archetype in the way that artillery is support, but I decided to instead go for a more balanced melee/ranged archetype as a sort of alternative to the standard fighters and barbarians.
I made the melee be more versatile than the ranged attacks so that they don't just rely on ranged attacks and can actually just get in there if they want to.
Keeping in mind both the Artificer's features and skills itself and those of this subclass, is this archetype balanced compared to the other currently published Artificer archetypes?
Combining machinery and magecraft, you forge a war-machine capable of taking down the toughest warriors.
When you choose this specialization at 1st level, you gain proficiency with smith's tools and have proficiency with your War-Body.
When you gain the means, you are able to create a mechanical servant but it cannot attack and can only be Small sized.
At 1st level, you forge a war-machine construct that can be donned and operated. A powerful contraption called a War-Body. The War-Body is the heart of your arsenal, upgrading the fierce construct as you progress.
The War-Body is a special weapon that you can don and doff as a piece of armor. The machine deals 1d10 + your Intelligence modifier bludgeoning damage and has a range of 10 feet. While inside your War-Body, you gain resistance to piercing and slashing damage.
The War-Body has a movement speed of 30 eet and a jump height of 5 feet.
Your War-Body has 1d6 + 8 hit points and has a base AC of 10 + your Dexterity modifier. Your War-Body does not gain benefits from shields. You may don armor on your person, but you will not gain additional benefits while inside your War-Body.
You do not take damage until your War-Body is destroyed. If your War-Body is destroyed with excess damage, the difference is subtracted from your own hit points.
If your War-Body is damaged, you must spend 5gp worth of materials and 1 hour to repair it. It regains 1d4 hit points + your Intelligence modifier. During repairs, this is not considered a Short Rest for you.
Your War-Body cannot be healed by spells and potions. Mend can also be used to repair an additional 1d4 hit point per short rest.
If the War-Body is destroyed, you must spend 25gp worth of materials and 8 hours to reconstruct it.
At 1st level, when inside your War-Body, you know the Thunderclap cantrip.
At 3rd level, you can cast self and touch range spells on your War-Body.
At 3rd level, you can upgrade your War-Body to be resistant either acid, cold, fire, lightning, or thunder damage.
At 6th level, you can add resistance to either necrotic or radiant damage.
At 6th level, you can upgrade your War-Body with a crossbow-like attachment that deals 2d6 + your Intelligence modifier piercing damage and has a range of 50 feet and a maximum range of 150 feet. The ammunition this takes is crossbow bolts.
At 9th level, you can upgrade your War-Body and make it a magical weapon for the purposes of bypassing immunities and resistances to non-magical weapons. Your War-Body also gains +5 AC.
At 14th level, you can use your melee weapon attack with the War-Body as a bonus action and as a reaction when a creature in range makes an attack against you.
At 17th level, you can take the Dash, and Disengage, actions as bonus actions. When you make a melee weapon attack against a creature with the War-Body, instead of rolling for an attack, it must make a Dexterity saving throw against a DC of 8 + your proficiency + your Dexterity modifier.