Now I know I'll get the classic "Wait for TEH GAME!" reply to this but I think it's worth discussing now. Not to praise or slam but to just discuss. According to what we know, each stage weapon will have it's own moveset and judging from the screenshots so far, there is an animation specific to picking up that weapon as opposed to it simply appearing in the character's hand like their personal weapons. In order for this to be balanced there needs to be a respective risk/reward to using the weapon.

If it ends up being low risk/high reward the stage weapons become broken and once someone grabs one the match may be over before it begins. I'd say in order to prevent the weapons from being overpowered or totally useless they'd have to be a high risk/high reward feature of the game.

From what I can gather, I'd guess the movesets would be rather simple. Pretty much restricted to the four face buttons with few if any combo possibilities. There would be a good reason for this. The stage weapons need to be significantly stronger than the default character weapons. Otherwise there'd be no point in using them. If they're that much stronger and have as complex a moveset they become broken.

But there also needs to be a risk in picking up the weapons. There can't be any safe frames during the pick up animation. If your opponent is then foolish enough to let you grab it they deserve to lose a chunk of life. Basically, this would possibly become useless in tourney level play then because in a tournament nobody wants to take risks. It would be high risk picking up the weapons and then even further risk being stuck with just four or five moves. At lower level or casual play it would be great because even with the limited moveset you get mad power. I don't know if this is how it'll end up but I can't see it being harmful if it doesn't break the game.