OSR: The Flame Pomerium (or Giant Mecha Fights)

This is the silliest idea ever, but I'm excited. This post was written in a hurry. Apologies for everything.

Warmachine, Ted Terranova

The Flame Pomerium 

When I was a youth, whenever I felt small or lonely, I'd look up at the stars. I wondered if the Authority was looking down on me. Turns out, He is either blind or cruel. When the Apocalypse came, it didn't come with fire from the sky and troops of angels. It came from the earth. From caves, from the sea, from all the deep and forgotten places of Creation. I was fifteen when the first Great Beast made land in Corpathium. By the time the combined armies of the Lemblics, Cratons, and Thurges cut its throat, six days and ten leagues later, three cities were destroyed. Tens of thousands of lives were lost. We mourned our dead and moved on. And then, six months later, the second attack hit Pilos. Then the third one hit Nahria. And then the fourth. This was no isolated event. The Beasts would not stop. This was just the beginning.

We needed a new weapon. The faithful came together, setting aside ancient conflicts and uniting against this new and terrible foe. To fight the Great Beasts, we created Beasts of our own. The Bulwark program was born. There were setbacks at first - the engines raised by the wizards proved too unwieldy for one soldier to control. A team of trained operators was required. We started winning, Bulwarks stopping Great Beasts everywhere. But the Bulwarks were only as good as their pilots. In the public eye we were heroes, saints, gods. Such hubris could not go unpunished for long...

Someone suggested a Mecha OSR game. I thought about it for a few seconds, sketched out the "usual" kind of system: upgrade slots, damage rules, a critical table. Then I realized I was being an idiot. Everything I could ever want was already there, in the rules. All I needed to do was change the scale.

The Flame Pomerium operates on 2 different scales.

Mortal Scale: your usual OSR game. I'd generally stick with simple classes: thief, fighter, wizard, knight/paladin. You might want to use race-as-class to add in elf and dwarf.

Bulwark Scale: 50x bigger, but the rules don't change.
 
To roll up a Bulwark (a giant, magically animated mecha made of wood, stone, iron, and clay), just roll up a character like you would normally. Its Int and Wis will be provided by the pilot(s). Casting classes may need to have the effects of their spells slightly adjusted. Some spells can be ignored entirely.

The re-scaling might result in some unusual situations and rules interactions. That's completely fine.

Thief Bulwarks are quick, stealthy, and light. They use small weapons and bows.
Fighter Bulwarks are heavier, designed to cut and slice.
Knight Bulwarks use heavy shield and armour, drawing the attention of their enemies.
Paladin Bulwarks draw on divine power.
Wizard Bulwarks cast dangerous, barely controlled spells from gigantic wand-cannons.
Elf Bulwarks dart in and out of combat, always on the move.
Dwarf Bulwarks are as sturdy as the mountains.

If you're not using race-as-class, there are a few options, depending on your system. You could reskin them as Roles (so Elf -> Scout, Dwarf -> Tank, Human -> Brawler, etc.). Or you could make them purely aesthetic. Whatever is easiest.

It takes 1-5 mortal-scale PCs to control a Bulwark. Roles can be filled with hirelings/NPCs as well.

Core OSR Changes:
The game is no longer about avoiding fights to get loot. The game is now about picking smart fights and fighting against impossible odds. It's not just about tactics - there are political considerations, ancient rivalries, deep mysteries, etc. Dungeons are probably out. Edits to Bulwark-scale spells and effects will be needed.

Interaction Between Scales:
Mortal-scale attacks can't damage anything on the Bulwark Scale.

Bulwark-scale attacks have trouble hitting mortal-scale targets precisely, but they automatically kill. If you need a number, 50d6 or more.

Reinhardt the Guardian, doctaword

Example:

It's just easier to sketch out this way.

3 players. They roll up standard characters:
-Bob the Fighter
-Cribbage the Wizard
-Lucinda the Thief

Personalities. Bob is the leader of the group. He's the charismatic, idealistic one. Cribbage is a little eccentric, an experimenter and a blunderer. Lucinda is skeptical but cunning.

They decide to roll up one Bulwark and operate it together. They chose to roll up a Wizard, and generate a standard Wizard with standard stats (excepting Int and Wis) and the standard spells. In this example, magic missile and mending.

Mending's effect, on a gigantic wooden construct, is adjudicated to be about the same as cure light wounds' effect. Magic missile is fine as is.

The GM the creates 2 other rival/allied Bulwarks: a slim bow-using Elf Bulwark, piloted by a group of androgynous mountain elves, and a Knight Bulwark piloted by Bob's older, wiser, and more successful brother.

The PC's Bulwark is named "The Pride of Basildon", the city that funded its creation. It is a massive wooden humanoid, three times as tall as a castle's highest tower, and bristling with wooden armour, pulleys, metal plates, and magical defenses. One arm ends in a wand-cannon made from a single lightning-struck oak and banded in purest silver. It can deliver a single devastating magic missile. The other arm ends in an articulated hand, but it can draw a dagger in an emergency.

Bob and Lucinda control the Bulwark from the head. During fights, they make collaborative decisions. They decide that they share command. The Bulwark uses Bob's Wisdom and
Lucinda's Intelligence. If Lucinda had sole command, then the Bulwark would use both her stats, but if she was ever knocked unconscious or killed Bob would be unable to help.
Cribbage works in the Bulkwark's chest, tending to its many mending-like spells. He is the only one who can activate the titanic magic missile cannon or deploy the mending spell to heal the wooden machine.

Monsters

Scale everything up. An lowly goblin becomes very, very frightening when it's 100' high. An Umber Hulk becomes the terror of the world.

Monsters don't have to be enemies either. A giant roc might be a faithful scout-hawk. A titanic griffon could be a mighty ally.

Again, don't scale stats or anything. Just descriptions, effects, goals, and needs. And the descriptions. Get bombastic. Run Michael Bay through a filter of Patrick Stuart.

Descriptions

Combat rounds at Bulwark scale should feel like they take longer. The GM is going to need to use more descriptive language than usual. Focus on:
-Movement: who steps where? Where do you get knocked/tossed? Every hit that deals more than 6 damage should result in knockback.
-Scale: remember, everything is 50x larger. A spell with a 100' range can now travel the better part of a mile. You have to dodge around villages and streams. A blow that would knock a mortal-scale character back 5' instead knocks the character half a city block backwards. 
-Ponderousness: swinging a sword at this scale is an epic endeavor that rattles the very foundations of the earth. The charge of a wyvern should feel biblical.

Damage

Rephrase your Death and Dismemberment rules as needed, to reflect damage to the Bulwark

Any critical damage hits the PCs unless they Save, doing 1d6+the critical damage inflicted on the Bulwark. For example, if the Bulwark was reduced from 10 HP to -4 HP by a 14-damage hit, each PC would need to Save or take 1d6+4 damage. Some damaging effects (acid breath, fire) might not allow a Save. As each PC is responsible for some critical aspect of the Bulkwark, the loss of one could doom the entire team.

Healing

Bulwarks heal on the same time scale as PCs, assuming an entire city devoted to repairs.
They heal on 1/10th the scale, assuming a village, and the active help of the PCs.
Unless the Bulwark is biological or otherwise alive, it never heals naturally.

Upgrades

Imagine a magic weapon on this scale. Cut it from the body of a fallen Great Beast. String your bow with dragon sinew, fire arrows tipped with the tears of lost gods.

But plate armor in a normal game and it's just an expense. Imagine the resources required to clad a giant in steel.

Every upgrade should feel suitably epic. Being able to cast burning hands is impressive when the hand is the size of a barn.

Other Stuff

This is just the sketch of an idea. If there's interest, I'll write up a full guide. Alternatively, steal this idea and run with it.