Looking for a rpg with a robust magic system

Check out O.L.D from the What's Old Is New mechanics. It's on ENWorld and is very solid.

Can you tell us something about that system?

It's based on d6 dice pools. I think there are free versions of it out there. It's surprisingly good. The OLD system is for fantasy games. You have your basic magic skills (Abjuration, Invocation, Divination etc.) then you use those in conjunction with a Magic Point economy to cause specific effects such as Abjuration (Mage Armor with 3 Soak) by manipulating duration, effect etc. Also it has a neat "Secret" system. THe kinds of magic you can do are limited by the Secrets you know. So your mage may have the Secrets of "Fire" and so they can access spells that utilize Fire.

The reason I got interested is that the Fantasy, Future and Modern editions are all interchangeable.

Here is the link. Check out the RESOURCES tab for a character builder and free resources.
WOIN (What's Old Is New)

And I found the rules reference there as well.

Rules Reference Document

This actually looks really good.