You should keep a UV Haze filter on every lens for protection.
A nice filter to have is a polarizing one – Use it when working with reflections in glass or sky with clouds.
A Neutral Density filter (or three) will be helpful when trying to get shallow depth of field outdoors on sunny days.
If working with a single, unchangeable lens, wide angle and telephoto adapters will be helpful.
There are more but these are your basic toolkit.
