Most advice/instructions seems to be for creating NPCs.
There's the
Weapon Tiers thread. A quick search of the Event Submissions forums didn't pull up any threads dedicated to accepted weapons. There are a few rejected and in progress, however.
I believe you just have to come up with a concept, a description (even better if you can fit it into the universe specifically) and probably flags (Bonus Hit Rate, Stun, Burn Chance, Whatever). As far as I know, the devs don't care what stats you give it, they've got a system to determine what is appropriate and will stick to it.
There's also the issue of obsolescence. You might want to create several different weapons to be spread out every couple of planets, so you always have one with relevant stats. Of course, then you run into not knowing what the next locations are and their themes.
If you just want to replace 'Rock', I've been saying Arm TFs with Claws should replace 'Rock'. Same damage, different description. Other characters can fight bare handed, after all.
I will say the Custom Shock Gear is currently one of the best weapons in the game. Not the highest damage, but Bonus Hit Rate + Stun Chance is incredible on Tech Specialists (two turns of Rapid Recharge) and Smugglers (Stun enables Sneak Attack), at least. Probably Mercenaries, too.