So my group is quit creative and would like to experiment with creating weapons or just bring different ones into the game. How do I do this and keep it fair?

Use the existing items as a base and try to keep similar items about the same in abilities and cost. You can also add rules since the weapons are "prototypes", so that if they roll really poorly (ie Despair or multiple threats) that something bad happens... maybe the weapon breaks, loses it's charge, blows up in their hands, etc.

Have the players give you their ideas and post em here. If it's too nutz or might cause some kind of cascade it's a good bet someone here will catch it.

With new equipment, new character abilities, or anything of the sort, the rule of thumb is this:

"If I were a player, would I choose this new equipment/ability/et cetera over a comparable option from the book?"

If the answer is clearly "Yes", then the new option is probably overpowered.

If the answer is clearly "No", then the new option is probably underpowered and should actually be beefed up.

If you're not sure how to answer - the new option and the official choices seem fairly equal - then the new option is probably well balanced.