Yeah, I can't say I'm too keen on any of the content other than Kenney's stuff (most of which I imagine is already here) but I like the concept of it, so it's worth supporting.

This is something I've always wanted (big suprise) and no, it's nothing like regular snapping.

The idea is something more like a drag-and-drop constraints interface. Specific types of parts can go in specific positions with angular constraints, something of an implicit skeleton.

A grid is fine if you want to assemble a thing out of regular size tiles, not much use if you want to assemble a mech out of variable size chassis, arms, legs, weapons, etc.

Blender doesn't even support interactive grid-snapping. What it has is increment snapping so you can snap an object to two increments to the left of its current position, but not to absolute grid possition (-2, 0, 0).

I imagine something like the following:

Component description format
Model description format
Model generation rules description format

Simple human/machine readable/editable formats based on JSON, YAML or somesuch.

Component description format file accompanies an image file of component sprites: describing geometry, connection and classification data for each component.
Model description format specifies a model assembled from components, describing the component heirarchy.
Model generation rules description format specifies further restrictions beyond the full set of permutations.

GUIs like you describe to help create component descriptions and model description would certainly be cool too.
 EDIT: Grr! How do you post code without the whitespace getting stripped? Component description example
[
{
"id": "wing1",
"bounds": [[0, 0,], [64, 64]],
"classification": {
"type": "wing",
"handedness": "left"
"mirrorable": "true",
"faction": "pirate"
}
"connectors": [
{
"id": "base",
"position": [64, 32],
"normal": [1, 0]
"required": "true"
"classification": {
"type": "wingSmall",
"handedness": "left"
"gender": "male",
}
},
{
"id": "weapon1",
"position": [0, 32],
"normal": [0, -1]
"required": "full"
"classification": {
"type": "weaponSmall",
"gender": "female",
}
}
]
}
]

Model description example
{
"id": "Billy's cool pirate ship",
"mirror": "true"
"root": {
"id": "chassis3",
"connections": [
"cockpit": {
"id": "cockpit76",
"connections": [
"weaponLeft": {
"id": "laserSmall",
}
]
}
"engine": {
"id": "engine4",
}
"wingLeft": {
"id": "wing1",
"connections": [
"weapon0": {
"id": "missilesCluster",
},
"weapon1": {
"id": "laserMedium",
}
]
}
]
}
}