This is fantastic
Any advice for adding new spells? Is there any existing magic systems that would port over easily?
The Magic Dice system is pulled straight out of GLoG, so I have some really good news for you! There are about a billion GLoG wizard schools, which should translate almost straight across, but may need some tuning for the system.
Here's a huge (but still incomplete) list: https://attnam.blogspot.com/2018/08/d50-glog-wizard-schools.html?
As long as you remember to scale spells with Dice and Sum, you can make just about anything work.