Script which modifies all fonts inside a given directory according to your desires. I can't think of any reasonable use cases for this.

fontkiller.py

#!/usr/bin/env python3

# The only requirement: fontTools (create venv, then pip install fontTools)

import os
from fontTools.ttLib import TTFont
from fontTools.pens.ttGlyphPen import TTGlyphPen

SOURCE_DIR = './fonts'
TARGET_DIR = './fonts_modified'

# Uncomment the mode you wish to use
MODE = 'NONE'
#MODE = 'ROT'
#MODE = 'BLANK'
#MODE = 'BLACK'
#MODE = 'SINGLE_CHAR'

ROTATIONS = 1 # only useful if mode is ROT
SINGLE_CHAR = '*'

def rotate_char(c):
    digit_map = {
        '0': '2', '1': '9', '2': '3', '3': '8', '4': '4', 
        '5': '7', '6': '5', '7': '6', '8': '1', '9': '0'
    }

    if 'A' <= c <= 'Z':
        return chr((ord(c) - ord('A') + ROTATIONS) % 26 + ord('A'))
    if 'a' <= c <= 'z':
        return chr((ord(c) - ord('a') + ROTATIONS) % 26 + ord('a'))
    if c in digit_map:
        return digit_map[c]
    return c

def rotate_all(input_dir, output_dir):
    if not os.path.exists(output_dir):
        os.makedirs(output_dir)

    for filename in os.listdir(input_dir):
        if filename.endswith((".ttf")):
            input_path = os.path.join(input_dir, filename)
            output_path = os.path.join(output_dir, filename)
            
            try:
                font = TTFont(input_path)

                if MODE == 'BLACK' or MODE == 'BLANK':
                    for table in ['gvar', 'fvar', 'STAT', 'HVAR', 'VVAR', 'kern', 'GPOS']:
                        # we have to remove these tables otherwise some fonts will not accept the new drawing
                        if table in font:
                            del font[table]

                    glyph_set = font.getGlyphSet()
                    glyf_table = font['glyf']
                    hmtx_table = font['hmtx']

                    box_width = 1000
                    box_height = 1200
                    
                    for glyph_name in font.getGlyphOrder():
                        pen = TTGlyphPen(glyph_set)

                        if MODE == 'BLACK':
                            pen.moveTo((0, -200))
                            pen.lineTo((box_width, -200))
                            pen.lineTo((box_width, box_height))
                            pen.lineTo((0, box_height))
                            pen.closePath()
                        
                        glyf_table[glyph_name] = pen.glyph()
                        hmtx_table[glyph_name] = (box_width, 0)
                elif MODE == 'ROT' or MODE == 'SINGLE_CHAR':
                    for table in font['cmap'].tables:
                        if table.platformID == 3: # Windows
                            new_cmap = {}
                            SPACE = 0x20
                            for code, glyph_name in table.cmap.items():
                                try:
                                    target_char = rotate_char(chr(code)) if MODE == 'ROT' or code == SPACE else SINGLE_CHAR
                                    target_code = ord(target_char)

                                    if target_code in table.cmap:
                                        new_cmap[code] = table.cmap[target_code]
                                    else:
                                        new_cmap[code] = glyph_name
                                except ValueError:
                                    new_cmap[code] = glyph_name
                            table.cmap = new_cmap
                else:
                    raise f'Invalid MODE selected ({mode}). Please set a MODE at the top of the file.'

                font.save(output_path)
                print(f"Processed: {filename}")
            except Exception as e:
                print(f"Failed to process {filename}: {e}")

rotate_all(SOURCE_DIR, TARGET_DIR)