Sure! Here's a test case generator for the given problem statement:

```python
import random

def generate_string(length):
    return ''.join(random.choice('ABC') for _ in range(length))

def tcgen():
    # Length of the string s
    length = random.randint(1, 200000)
    # Generate string of specified length consisting of characters A, B, and C
    s = generate_string(length)
    return s

# Example usage:
s = tcgen()
print(s)
```

This script generates a test case for the problem where you need to find the maximum number of operations to replace "ABC" with "BCA". The test case consists of a string `s` with a length between 1 and 200,000 containing only characters 'A', 'B', and 'C'. The `generate_string` function helps in creating a random string of the specified length.