First, there is suggestion for new empire.
Now, there is suggestions for Tier 2 Units!
Anyway here is the following:

Miner=>Scavenger
Cost: 450 G to Upgrade
Cost: (150/0/2)=>(300/0/3)
Double the mining amount for miners, but increase in pop cost.
Health increases by 50%, and damage increase by 50%.

Swordwrath=>Swordmaster
Cost: 300 G to upgrade
Cost: (150/0/1)=>(200/0/1)
Increase the swordwrath health by 50%, damage by 50%, slower movement speed.
Addition of small armor, increasing normal arrows resistance.

Archidons=>Ranger
Cost: 350G to upgrade
Cost: (350/0/2)=>(400/100/3)
Increase range by 25%, damage by 25%, and more movement and attack speed.
Also addition of small armor, and health doubles.

Speartons=> Phalanx
Cost: 450G and 50 Mana to upgrade
Cost: (450/50/3)=>(500 Gold/100/3)
The speartons block ability now reduces 50% instead 40%, and shield bash can be used without blocking stance.
Increase 10% in damage.

Special Abilities:
Phallanx Mode (6 Phalanx needed)[200 Gold 100 Mana]:If there are 6 Phalanx in a line, it could form Phalanx mode.
During this mode, nothing can move the Phalanx without them breaking the stance. Also units unable to move past them.
Cost: 200 Gold 100 Mana

Shadowraths=> Assassins
Cost: 450 Gold and 150 Mana to upgrade.
Cost: (450/150/4)=>(600/200/5)
The Assassin have a 15% health bonus, 15% more damage, and increase in movement and attack speed.

Special Abilities:
Magic killer(Passive): Deal 50% bonus damage on magic users, causing Medusa and Marrowkais to die in a single hit.
Shinobi 3(200 Gold 200 Mana): The Assassins do sadistic training, causing the stealth duration increase by 2 sec, and massive damage bonus.

Albowtross=> Flying Machine
Cost: 400 Gold and 200 Mana to upgrade
Cost: (400/200/4)=>(250/100/2)
The Flying Machine have 50% reduced health and 30% reduced damage, causing them have same stats as Archidons.
Increase in movement speed (As fast as Eclipsor), increase in attack speed, and have a high armor (Same as Deads/Spearton).

Special Abilities:
Flak Cannon (200 Gold 100 Mana): The Flying Machine shoot a single shot of Flack Cannon, dealing small splash damage.

Merics=> Priest
Cost: 300 Gold and 200 Mana to upgrade.
Cost: (300/200/3)=>(350/250/4)
Priest have 30% increase in health (4 bars) and increase in movement and attack speed.
Also increase 15% health on healing, and reduce healing cooldown by 0.5 sec (become 2.5 sec)

Special Abilities:
Aura of Healing(300 Gold 200 Mana): Heal the unit in aura every 5 seconds, with heal amounts of a meric.

Magikill=> Magus
Cost: 500 Gold 400 Mana to upgrade.
Cost: (500/400/5)=>(400/500/5)
Increase in movement speed, and a small armor is added.
Also, Magus don't have any physical attack, so he doesn't go forward to face the enemy.

Special Abilities:
Aura of Magic (200 Gold 200 Mana): Any unit in the aura receive 50% increased damage from magic (Allies too)
Ice Break(100 Gold 200 Mana): Summon a sharp Icicles from the ground, stabbing any unit stand on it. (Usage of 20 mana)
Golem (200 Gold 200 Mana): The Magus sacrifice himself, however, he call upon a golem that takes over his space. (Usage of Magus's life)

Golem Abilities:
Ground Stomp: Stomp all enemy in the ground, stunning them for 2 sec.
Earth Shield: The Golem taunt every enemy in the battlefield, focusing them on him. Reduces damage taken by 50%, but unable to move.

Enslaved Giant=>Enslaved Ogre
Cost: 750 Gold to upgrade
Cost: (1500/0/7)=>(1800/200/9)
Increase in attack speed, movement speed.
Health increased by 15%, and addition of a medium armor (same as Deads)

Special Abilities:
Roar(200 Gold 200 Mana): In 20% below health, the Ogre roared in pain, stunning enemy units for 3 sec.
Stomp (300 Gold 300 Mana): Stomp an unit to death, killing him instantly.(Only works against ground, non-Ogre/Giant units)

Info: If you already make some units, and purchased the upgrade, the units you already make doesn't change.
More Info: Upgrade only works AFTER you purchased it, and once you purchased it, the units you make will be different.

P.S: This is just a suggestion.

I don't really like the idea. It sorta overshadows the original races, which I prefer more.

Anyways, I'd rather have unique unit abilities instead of unit upgrades.

Quote from Juice Box

I don't really like the idea. It sorta overshadows the original races, which I prefer more.

Anyways, I'd rather have unique unit abilities instead of unit upgrades.

Agreed, I like the original units better XD
Swordwrath sounds cooler than swordmaster lol

The original idea is better...

Well, this is just a suggestion.
It's also include strategy ya know :/

I was just reminded of something. It's an old story, I'll put a Stick Empires twist on it.

The Arrows and the Shield

The greatest Spearton to ever exist, Leonidan, had a completely unmarked body. Never was his body wounded, for his armor was crafted of the strongest metal ever found. There is only 48 ounces of this metal, Machimatite, and it has saved his life countless times. Enemies soon feared Leonidan so much, they cowered under his massive build and shining armor. The last thing any enemy of Leonidan saw, before eternal darkness, was his fiery blade. His armor is legendary, and now retired, the Spearton intends to sell it to gain control of the conquered Chaos Empire.

The greatest Archidon to exist, Boarsit, could shoot an oakleaf as it fell from a tree, hitting it exactly through the middle, from 1000 meters away. His bow was legendary, the string with a tightness requiring 200 pounds to string. But more legendary were his hand-crafted arrows. Rumors say that it comes from an extinct tree, 20 meters in diameter and a mile in length. "There is nothing that my arrows cannot pierce!", Boarsit bellows as they win the battle. Each arrow is so sharp, it passes through 10 men before coming to a stop. He too, retired, plans to sell it.

Boarsit and Leonidan meet at the same square. "WE WANT THE CHAOS EMPIRE IN RETURN FOR..." they boom. "My arrows can pierce through half Earth's volume, nothing can stop them!" Boarsit says.

"My armor can withstand any amount of weight, nothing can surpass it!" Leonidan bellows.

A watcher, not intending to purchase either, smirks and asks,"Then perhaps, Boarsit, you would like to shoot an arrow at Leonidan's armor and see what happens?"

Some of these effects are detrimental, so they aren't really upgrades that should be permanent. A population cost increase in itself is detrimental to the max late game.

Upgrades that are permanent should be purely beneficial. If it has some bad sides to it, you should be able to make a unit without the upgrade.

Quote from Juice Box

This is gonna sound stupid, but...

What happens next?

Seriously.

According to what I hear, the two of them did that, and well, kaboom!

Quote from Juice Box

This is gonna sound stupid, but...

What happens next?

Seriously.

It's the same when Nokia 3310 exploded.
Also why this thing is off-topic?
Anyway, Tier 2 units is just a suggestion, so players could have more strategy.