Ok dumb compression concept: Hash-breaking. Replace every 32 bytes with 8 bytes of hash, 'decompressing' is simply brute-forcing a match. Consistent 75% compression ratio but really slow decompression, and you'd need to be able to tell the decompressor to retry a failed attempt, maybe have it store the hash of each file so it can at least guess where it went wrong (other than "everywhere"), though that can easily tank the ratio.