It would be good if we could create our own motion graphic titles indeed.
I have not found a way yet, but PD enables you to modify existing motion titles and save them. They will then become available under the custom tab.
PD stores the new titles in: c:\Users\%userdname%\Documents\Cyberlink\PowerDirector\19.0\MyTitles
Each title in a new folder with a sequence number starting with 000 (Title_000]
If you look inside the .CLT file, you see the XML parameters for the moving title. You can try and change these, but obviously it is easier and safer to have software that can help you with that.
If not already, maybe an idea for Cyberlink, to make this software available and have members to upload their creations in the DirectorZone?