I think "keeping magic in balance" and designing it to be "modular where you can group different elements together to create your own unique spells", are mutually exclusive concepts.
Add this on top of very flexible racial/class construction and I think you've created a pool of variables you likely can't insure proper balance for. Now there isn't anything particularly wrong with systems that are out of balance, just that the onus of balancing opposing forces for a game falls on the shoulders of the participants or a GM, and not the rule system. If there are unaccounted for "broken" combinations of abilities and effects in your system, players will intentionally or unintentionally discover them. This can cause disappointment and a lack of "faith" in your rules.
I like fantasy games that equally value melee, missile, and magic forms of combat. If I am given options I want to be assured my choices in any given direction are reasonably valid against any other combination. I do not like any of those three main concepts to be designed to be a lesser option unless I am very clearly told that is so within the rules; before I buy and paint models; before I play games or convince others to play games.