With damage leak x% ~, and block limit (x) as well as a (chance) for guard break (next hit on you does x% damage more) and stuns you for x ms. Breaking guard is also dependent on power of spell that hits you. And certain enemies break guard automatically (rhinos)
Why would I write how to possibly balance block if people are against even considering a proper block and tell me the 3-second-to-drain-your-mana-empty shield is an alternative to a guard/block function. Clearly we are at a rather premature stage of the discussion about a block function to consider details ;P
Needless to say, if block is considered, the details are open for debate. But block is open for debate as well, so no point detailing specifics.
By the way, a block function is fundamentally different from a sustained shield function. The shield has no dynamics, you activate it, it is active.. and it remains active for x amount of time. I find that rather mhh, boring.
If a block function was to act like a magical directional shield for one, I could hold it for fun, it'd look cool, i could prepare myself when jump down somewhere holding block while falling and in combat I could cast 3 or 4 spells, jump in a good position and block the counter fire then just jump back in the fray = adds fluidity.
So yeah, my suggestions usually do not come out of nowhere ;P I am just a bit lazy writing this down in a topic thats not... ehm, about blocks. ^^