I suppose the term free-form structured magic system is very contradicting lol. What I mean is People could use whatever kind of magic they want, with the exception for game breaking magics, but they need to define a system for how it works and what the magic's source is. They need to layout the rules, the weaknesses and everything so that players can come up with strategies for how to deal with the magic. 

In regards to the how much is too much topic. I was more so thinking how to define the limitations. Like how much magic can I use before I go insane? How does one measure it? .... You knooooooooooow.... I'm over thinking things aren't I?