1- we do not save any private key and only ask for the private key when the user decides to send some coin
Did you mean that your application will work as a tool to generate offline private keys" such as paper wallets?"
Also, does this mean that it will not support segwit[bc1] "have a specific formula for the private key"?
Generally, the approach that you are going to do is oriented to a lot of wallets so you have to do a lot of hard work to prove yourself in the market.
what do you think? is there any app like this?
If I understand correctly, exodus does the same.