There are actually benefits to both specialising and diversifying your character. Specialising your character will allow your to unlock better skills giving greater damage/healing/etc., whereas diversifying may restrict you from getting those top-end skills, but allows you choose a varierty of others to enhance you in other ways.
For example, if you specialised in the Assassin type you could unlock some really brutal top-end attacks. You may decide to lean into the Paladin type instead, sacrificing those attacks to become an Assassin-Paladin hybrid. This would allow you to backup your firepower with the help of some white magic. How much you lean from one to another is your choice. You could even lean into 3 or 4 types, although you start to saturate the benefits at that point.
At this point mixing doesn't give hybrid abilities (like a healer who does melee being better at healing melee injuries), but it's definitely something we should consider. I like your idea!