i think the private key of Bitcoin should not be a fixed 256 bit. it should be able to change in an interval. the user can select the number of digits of its private key. For example: between 500 characters and 256 characters.
Why is it a public number: 256 bits?
The character of the private key is a privacy. The number of digits in the private key should also be a privacy.
First, you should know that cryptography is a very-very serious field, many computer scientists and mathematicians have devoted their lives to study it. There are huge incentives to break cryptography, so if someone will indeed find a flaw, it won't be an amateur like you. You should relax about the fundamentals and instead spend your time how to secure other things.
And to answer your questions, zero bits are still bits. According to Bitcoin wiki, keys starting from 0x01 are valid, so they don't have to literally be 256 non-zero bits. And your proposal of "varying key length" would do absolutely nothing, because for all purposes keys would just be padded with zeroes to maximum key length.