This will allow players to fix incompatibilities of mods that use the same collision mask.
To do this add this to setting.lua:

data:extend{
{
type = "int-setting",
name = "name-of-option",
setting_type = "startup",
minimum_value = 11,
maximum_value = 15,
default_value = 12,
order = "a-a",
}
}

Not sure I understand, can you give an example where someone needs to reduce the collision mask?

I have a mod that uses collision mask 12 and 13. If any other mod uses those collision masks then they are incompatible so they can't use both mods at the same time. If there is an option to change the collision mask the player could then fix the incompatibility and play with both mods at the same time.