<p>I think that a cut-and-come-again true perennial vegetable for a non-tropical climate will need to be one with an indeterminate growth habit, whose leaves do not turn bitter when it blooms. A few of us have been talking about it; some of the hardier members of the genus Hibiscus might be a possibility. What do you think?</p>
There's one selected for centuries as a perennial leaf vegetable ... mulberry. Ok, it's mostly used to feed worms, but it's very productive, nutritious, edible, and some varieties taste very good. Season can be quite long if you keep picking shoots.
Another very good one is Tilia trees. Tilia cordata is very good.
There are vegetable varieties of wolfberry.
As lower growing there are many to choose from ... campanulas, chicory, good king Henry, mitsuba, some sedums, sorrels, sweet cicely, turkish rocket, violets ... and bamboo (seasonal).
Hibiscus? I don't think they are that great to be worth the efford. Crossing with hardy relatives and getting something good and edible won't be easy.