To go for an arch pergola could definitely be a good idea, especially if you don't want anything too harsh or rigid. With the fluid shapes of an arch it's easy to create a very romantic and soft impression in your garden or outdoor space. Many arch pergolas are well made and they come in many different styles, there is not just one type to choose between.
For instance, below you can see two very different examples of arch pergolas: one in a very fine wood which features more of a wind protection on one side and an open feel on the other three sides. This would be a great choice if you really appreciate the style and look of wood and you don't mind doing a bit of maintenance every once in a while.
Then you also have the metal arch pergola below: this Sean Conway arc pergola is so romantic and elegant. If you have been looking for a centerpiece in the garden either for a wedding, and event or just as a focal point to work around, then you would just have to see this one. It features exquisite scroll work and very fine lattice details. These two pergolas are indeed very different, but both would be worth a look if you're thinking about getting a structure for your garden. There are of course also other materials to check out such as vinyl, other types of metal as well as other types of wood, but these two below are a good representation of what's available out there.
"If you are looking for classic luxury and an incredible addition to your backyard, the Premium Arc Pergola, manufactured by VIFAH, is the product to enjoy for years. With a warm color throughout its rich wood grains, the Premium Arc Pergola gives your backyard an elegant design yet a warm inviting character. It also provides a focal point for decoration with climbing plants, hanging baskets, and occasional decorative items such as lights, ribbon, or streamers. The pergola is made of 100% highest grade of Asia Pacific Mahogany (or sometimes referred as Premium Balau or Premium Shorea), an extremely durable hardwood species that is known for its long-lasting beauty, durability, and extra strength. For centuries, it has been the premium wood of choice for building ships, furniture, decks, and houses. Benefits of Asia Pacific Mahogany"
Sean Conway Normandie Pergola
"Lend a Decorative Element to Your Yard with this Classic Square Pergola. Anchors in the Ground or Stands on a Flat Surface. Frame Made of Steel. Powder-Coated Black Finish. Accented with Charming Lattice-Work Design. Reinforced Corrosion-Resistant Galvanized Hardware. Rust-Resistant Treatment for Better Weathering"
These pergolas can be found at amazon.com