When megaman defeats a robot boss, he gains a new weapon. This process involves strenuous battle with the robot boss until it is destroyed; then and only then does megaman gain the new weapon. This is quite different from kirby, in that all kirby has to do is inhale the enemy of choice and then literally absorb their power.
However, while I don't think it's right to call megaman's trait "power stealing", I don't think that ability will make it into brawl nonetheless. I think megaman's attacks will have specific "megaman universe" boss powers mapped to them, and they will always stay the same. For example, samus's over+b is a missle, whereas megamans might be a crash bomb. Samus' up+b is the spin attack; megaman's might be rush, beat, or boot jets (depending on if it's classic or X). Samus's neutral b is her plasma cannon, obviously megaman's would be the buster. That's how I think it'll go down, anyways.