For now, the mnemonic words is totally random which is hard for people to memorize.
You aren't supposed to memorize it.
So，how to store the seed?
Write it down on paper. Job done.
If you are desperate to store your seed phrase electronically, then encrypt it using a standard encryption method. Any time someone tries to come up with their own weird method such as this, they either make an error they haven't considered and so their whole system is so weak as to be easily attacked, or they make things so overly complicated that they can't actually recover their seed phrase and their coins at all.
In your system you need to store both your customized seed phrase and your customized word list file. Where are you planning to store both of these things? If you can't store them securely, then your system can be broken. If you can store them securely, then your system is unnecessary since you can just store a (optionally encrypted) seed phrase and passphrase instead.
Hi, for the encryption algorithm problem, I really suggest you to try the algorithm. It's a very easy encryption algorithm. And we cannot stop trying just because of the possible risks, right? I would be very appreciated it, if anyone find the loophole on it.
For the storage problem, I think it's a trade off. There's no absolutely safe space to store the seed(including paper). The customized phrase is stored in your brain(that's very important.). The word list file can be stored in cloud or hard disk. If you set a strong customized seed phrase, it's would very hard to get the seed just using the word list file(cause it's just a random file, and don't contain any informations). There's no general way to crack the file, the only general way is brute force. And I keep brute force difficulty same as 2^128. For now, it's impossible.