Hi
I know someone who took photos of injuries following apparent rough handling by BT police. When the claimant went to his solicitor he requested the photographer gave evidence in court as to the time date and place the photos were taken. If you were to offer this service you might want to consider if the images would be considered evidence in their own right or if you might become a witness in some form as this could make it more trouble than there worth.
If you want a niche how about macro photos of Jewellery and similar items especially those items with serial numbers. Kept in a safe such photos could help with truculent insurance companies in the event of burglary, fire or other loss. Just a thought.