[sp332]

There are several such projects - swfdec, lightspark, gnash, etc. From what I've seen of them, they all underestimated the amount of effort. Most of them just petered out. So if you're going for it, I would worry less about duplicating effort and more about setting realistic goals for the project.

[pluma]

The big problem is that AS2 and AS3 are drastically different from each other and neither of them can safely be ignored.

Supporting even one of them in its entirety is a Herculean effort. Supporting both is insane.