It’s a good challenge to develop a password manager, as you’ve already been told, you’ll have to focus on the security part which is essential for this type of tool.
Personally I use KeePass for my passwords. You may want to study their way of dealing with the security. Maybe you can even use the same technology as keepass for the encryption of your database (keepass is open source). It would have several advantages: you would be sure to use a safe technology, your users would be confident, your users could easily migrate from keepass to PassGenius and vice versa.