An intelligent user can create seed himself or add any of words and symbols as entropy.
You can and should not ever rely on the user to be the source of entropy, they will never create strong ones ever.Quote
As an option, I will add the ability to use any arbitrary file as a seed, and multiple hash (>100000) to slow down brute-force, this will be really reliable, I believe. 
This makes your program to be a brainwallet which are very unsafe and 100k hashes does not make brute forcing slow....and multiple hash (>100000) to slow down brute-force, this will be really reliable, I believe.
I don't think that will be effective in the long run. An Nvidia V100 can theoretically do 10x as many hashes as a K520 and they were released just 3 years apart. So I imagine if not the current Ampere cards or next year's cards, one of these years someone's going to be able to pass 100K hashes/s at the same speed people pass 2000 hashes/s (number of PBKDF2 rounds of seed phrase) today.