I love that Boomerang idea, although it might be a bit overpowered as it looks to be the ultimate deck-out prevention tool. Very cool idea though.
I just made a thread for discussion about the boomerang.
But I think of a fix for being "the ultimate deck-out prevention tool".
Boomerang can be targeted by ANY target-able spell or ability(e.g Heal, Mutate, Thunderbolt, Drain Life, Bless, Quicksand & etc). Upon being targeted, boomerang is destroyed. Does not apply for "Steal" (card & ability), "Improved Steal", "Flying Weapon" or "Animate Weapon".
All elements have targeting spells or abilities. Boomerang is vulnerable to them even if they are not permanent destruction spells/abilities.
Discussion thread:
http://elementscommunity.org/forum/index.php/topic,3860.0.html