Here's a rather neat blast from the past. In 1994, Rockman X manga artist Yoshihiro Iwamoto offered his readers a chance to have their weapon designs immortalized in then up and coming new chapters. The contest, entitled "Iwamoto's Weapons Lab", accumulated a handful of entries with weapons ranging from the sublime to the obscure. Scans of these submissions below.
Weapons featured on the left most scan include an environmental friendly rainbow buster, a rocket buster that flies X to safety in sticky situations, a bat buster that shoots bats(?) and my personal favorite, the quintuple X-Buster. The other three scans focus on Iwamoto's favorites and finalists: the Angel Buster (shots turn enemies into friends instead of obliterating them), the Cyber Fist (solar powered, hand-to-hand combat weapon), and the Laser (shoots a powerful beam, supports beam sabers and can create a powerful magnetic shield). Most didn't end up in the actual comic, but did earn the right to be featured within the pages of Comic Bom Bom alongside commentary from Iwamoto himself. The winning design(s) received an autographed copy of the manga's latest collected volume and a copy of Rockman X for the Super Famicom.
Needless to say, there's some pretty interesting concepts going on here. It's unfortunate these type of contests have largely been exclusive to Japan as I'm sure international fans' creativity could certainly yield some awesome results.