One interesting thing to think about when making a system like this is to decide whether you want to (a) try to create a system that accurately matches the existing powers or (b) try to create a system that creates powers as balanced as possible. For example in the existing system a -SAM to AC is worth the same as a -2 to AC, and based on what I've seen of the powers, that accurately reflects how powers are designed. But SAMs can be significantly higher than 2 - especially in the higher tiers. And multiple attacks also tend to be undervalued by this system, because at high levels you get lots of static modifiers to damage so even a relatively weak secondary can become another opportunity to apply these powers. But of course even in the existing powers, multiple attack powers are often agreed to be superior to single attack powers. So how should the systm reflect this?