Your engagement ring should be something that reflects your personality. It is something you will most likely wear every day of your life for a very long time!
Custom engagement rings can be made to match a number of designs you see in your favorite antique shop or they can be based around a new design that you have dreamed up since you were a child. It is best to go into the jewelers with at least an idea of what you want. Take some designs you want to draw inspiration from and the jeweler can help determine a unique way to make sure you get a little bit of what you like about each.
Also, don’t be pushed into something you really don’t want. The point of a custom engagement ring is that it is your custom engagement ring! Make sure it is perfect before you decide to move forward with the design process.