Metal reinforcing plates exist that allow you to strengthen broken or weak doors, right in the area where the latches and locks are installed. This material was from Schlage Locks.
I also showed a neat way to use a hole saw to cut a hole just slightly offset from an existing hole, something usually very hard to do cleanly. Simply drill a hole in a piece of plywood with the hole saw. Then clamp this guide to the door where you want the new hole. Even if there is nothing for the pilot drill to set into, the plywood cutout will guide the hole saw through the rest of the door. Why would want to go to all this trouble? Maybe you want to install a handle or lock that has a different off-set than the existing one.