Freeform wouldn’t work as my world has rules that govern how it works. The thing with my setting is that there’s a few requirements that need a framework. Specifically the style of combat employed by the Tali’Centi and how magic is governed. I have pretty strict guidelines for both those in my writing and thus would require a ruled framework in an RPG to do likewise (for my writing it was done for both consistency in magic use and to place some hard limitations on what could be accomplished with it -I can’t stand fantasy where magic is a constant deus ex machina).
The two systems I’m most familiar with are both d20 systems. I like them both a lot but neither will work (particularly the magic power creep doesn’t work AT ALL for my setting). I thought about creating a few custom classes for an Epic6-style campaign but without re-calculating the underlying math it would be a crapshoot as to whether or not the system is balanced.
D&D Next would be a little better since bounded accuracy keeps the power creep in line, but my setting simply isn’t that high fantasy so it would be a lot of work. The near elimination of feat trees also makes that less desirable.
And other than that, I just don’t have the time to learn another system to the point where I can make significant changes to it and understand the ‘under-the-hood’ impact that will have on it.
I’ve started a rough framework for an essentially classless system, but I think it will remain a sideshow hobby project rather than an actual playable system.