UPDATE: Please use https://github.com/mathieudutour/sketch-file since this script was just a quick proof of concept and is not very futureproof. If you still decide to go ahead, this script will create a new, blank Sketch file, with proper UUIDs so you don't get a conflict creating multiple libraries from the same blank file)

newsketchfile.rb

#!/usr/bin/env ruby
# Generate empty .sketch file, with brand new UUID

require 'fileutils'

DOC_UUID = `uuidgen`.strip
PAGE_UUID = `uuidgen`.strip

DOCUMENT_JSON = <<~HEREDOC
{
    "_class": "document",
    "do_objectID": "#{DOC_UUID}",
    "assets": {
        "_class": "assetCollection",
        "colors": [

        ],
        "gradients": [

        ],
        "imageCollection": {
            "_class": "imageCollection",
            "images": {
            }
        },
        "images": [

        ]
    },
    "colorSpace": 0,
    "currentPageIndex": 0,
    "foreignLayerStyles": [

    ],
    "foreignSymbols": [

    ],
    "foreignTextStyles": [

    ],
    "layerStyles": {
        "_class": "sharedStyleContainer",
        "objects": [

        ]
    },
    "layerSymbols": {
        "_class": "symbolContainer",
        "objects": [

        ]
    },
    "layerTextStyles": {
        "_class": "sharedTextStyleContainer",
        "objects": [

        ]
    },
    "pages": [
        {
            "_class": "MSJSONFileReference",
            "_ref_class": "MSImmutablePage",
            "_ref": "pages/#{PAGE_UUID}"
        }
    ]
}
HEREDOC

META_JSON = <<~HEREDOC
{
    "commit": "120f488f43e85cae5c74eeae99d970dcf7fe443d",
    "pagesAndArtboards": {
        "#{PAGE_UUID}": {
            "name": "Page 1",
            "artboards": {
            }
        }
    },
    "version": 105,
    "fonts": [

    ],
    "compatibilityVersion": 99,
    "app": "com.bohemiancoding.sketch3",
    "autosaved": 0,
    "variant": "NONAPPSTORE",
    "created": {
        "commit": "120f488f43e85cae5c74eeae99d970dcf7fe443d",
        "appVersion": "51.2",
        "build": 57519,
        "app": "com.bohemiancoding.sketch3",
        "compatibilityVersion": 99,
        "version": 105,
        "variant": "NONAPPSTORE"
    },
    "saveHistory": [
        "NONAPPSTORE.57519"
    ],
    "appVersion": "51.2",
    "build": 57519
}
HEREDOC

USER_JSON = <<~HEREDOC
{
    "document": {
        "pageListHeight": 110
    },
    "#{PAGE_UUID}": {
        "scrollOrigin": "{26, 26}",
        "zoomValue": 1
    }
}
HEREDOC

PAGE_JSON = <<~HEREDOC
{
    "_class": "page",
    "do_objectID": "#{PAGE_UUID}",
    "booleanOperation": -1,
    "exportOptions": {
        "_class": "exportOptions",
        "exportFormats": [

        ],
        "includedLayerIds": [

        ],
        "layerOptions": 0,
        "shouldTrim": false
    },
    "frame": {
        "_class": "rect",
        "constrainProportions": false,
        "height": 0,
        "width": 0,
        "x": 0,
        "y": 0
    },
    "isFixedToViewport": false,
    "isFlippedHorizontal": false,
    "isFlippedVertical": false,
    "isLocked": false,
    "isVisible": true,
    "layerListExpandedType": 0,
    "name": "Page 1",
    "nameIsFixed": false,
    "resizingConstraint": 63,
    "resizingType": 0,
    "rotation": 0,
    "shouldBreakMaskChain": false,
    "style": {
        "_class": "style",
        "endMarkerType": 0,
        "miterLimit": 10,
        "startMarkerType": 0,
        "windingRule": 1
    },
    "hasClickThrough": true,
    "layers": [

    ],
    "horizontalRulerData": {
        "_class": "rulerData",
        "base": 0,
        "guides": [

        ]
    },
    "includeInCloudUpload": true,
    "verticalRulerData": {
        "_class": "rulerData",
        "base": 0,
        "guides": [

        ]
    }
}
HEREDOC

File.open("document.json", "w") { |file| file << DOCUMENT_JSON }
File.open("meta.json", "w") { |file| file << META_JSON }
File.open("user.json", "w") { |file| file << USER_JSON }
FileUtils.mkdir_p("pages")
File.open("pages/#{PAGE_UUID}.json", "w") { |file| file << PAGE_JSON }

%x(zip blank.sketch -r pages document.json meta.json user.json)
%x(rm -r pages document.json meta.json user.json)