The team-swap is an interesting twist on the converter's attack, and the roller is a good candidate for it as it requires the activation to prevent it being invincible.
I do wonder if there's other applications though. A reverse-converter would get insta-murdered mostly, but for key puzzles it might be relevant.
Or if one wanted a flaming barrel type of attack, then maybe there's room for a piece that spawns bullets when it has an empty line to an enemy.
I fear the aggressive roller would actually be more of a friendly roller as it would just get punched back to you, given that rollers only die when hit from behind when rolling. Or maybe the variant wouldn't and it would die on its first bump - then you'd get your exploding flaming barrel. I will probably try out a few variations as there are indeed a few loose threads to pull at here.