Answer: I am assuming these are also in containers. Clematis should be fully hardy in your zone if planted in the ground. It is possible they will overwinter for you, it is not unusual for them to look quite unpromising by spring. Be very patient before deciding they are dead. The rule of thumb with clematis (as with most perennial vines) is that the first year they sleep, second year they creep, third year they leap. In other words, they are slow to establish and begin performing at their full potential. I would not recommend crowding them to achieve a fuller look as this would simply stress them.
Annual vines are the fastest growing and with annuals you would not need to worry about overwintering as they die with the first fall frost anyway. You might enjoy moonflowers (Ipomoea alba -- evening flowering, huge white fragrant blooms), black eyed Susan vine (Thunbergia, grows to about five feet and so is perfect for a large container), the sweet potato vines (grown for their colorful foliage rather than blooms, terrific trailing downward spilling out of the container), purple hyacinth bean (Lablab purpurea, Dolichos lablab) and Asarina (a smaller, delicate textured vine). You can grow all of these yourself from seed, plant the seed directly in the container outdoors.
I hope this gives you some ideas.
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