Answer: This is a very interesting project and will take some thought and planning as to what to include and what to try to represent in some way because for example it can't actually be grown in your climate or you simply do not have space to grow it.
Eucalyptus for example is not hardy but it can be grown as an annual, it is often sold in the spring as small plants. Dates grow on date palm trees (these are huge 80 foot trees and need a much warmer climate such as southern California) but perhaps you could use a potted palm which you overwinter indoors as a houseplant. Alternatively, perhaps you could use a garden plaque picturing a date palm to indicate its presence without actually planting one.
Broom (Cytisus) should be hardy for you, to grow it requires full sun and a very well drained location. Here is some information about it you may find helpful.
There are many different ways of interpreting your theme. The following article discusses this and provides links to a church garden in Missouri where they seem to be growing many different annuals and perennials. You might find that useful as a reference.
You might find some ideas at the Cox Arboretum garden in Ohio, if a particular plant grows for them you should be able to grow it.
Here are some more with plants you should be able to grow. One in Michigan
Another in Ohio
One in Ontario
These examples show just a few of the many ways you could interpret your theme using plants that grow well in your local area.
There is also a web site devoted to the subject that you might find interesting while you are planning, it even includes a section on plants and their substitutes.
Good luck with your project! I hope this helps.
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