We never actually set limits on what they could do and stuff like that, so I'd like to suggest a plan and encourage other ideas and comments.
Essentially, non-zoan devil-fruit weapons must have an area of effect or a trigger. The outside of something, the spike on the glove, etc etc would be area of effect. Trigger would be such as the trigger on a bullet, or when you hit something with the blunt of a blade.
Now, for the abilities. Non-zoan devil-fruit weapons will never be able to grow in power. The ability can not be used in a manner not specified. No, techniques adapted to the ability are different, but that really depends on the fruit and weapon. That is a more case-by-case problem. Only one part of the ability may be used. Logias create and control, so the weapon may only create or mimic the effects of the created substance. Paramecias become something, so either a certain part is always that thing or there is a trigger to make it transform or activate etc.
Next, I'd say that devil fruit weapons here on out would require history as to improve the quality.
Finally, I'd say that the more powerful the weapon, the more difficult to use. Adding some sort of drawback or drawbacks to make it a weakness as well as a strength.