Ah Ha!, You do have some experience. It is great to hear someone put their "sold as indoor bonsai" juniper outside and it survived. Well done.
Deciduous trees, maples in particular are fun because of the color changes in autumn. Amur maple will do you well. The
Korean maple, Acer pseudosieboldianum, is a good choice but it is difficult to source. You need to make sure it was either cutting grown or grown from seed. You should avoid a grafted Korean maple. Reason is, the under stock might be regular Japanese maple, which would NOT be cold hardy.
Look around your neighborhood. What deciduous trees & shrubs are being sold for landscape use? Which ones have good autumn colors? Crab apples make decent bonsai if you get cutting grown cultivars. They are at least as hardy as your juniper. The leaf colors are not the best in autumn, but the colorful fruit is a great autumn interest. Also check out Cotoneaster. A few of the cotoneasters are quite winter hardy and should be available at your local landscape nursery.
For cutting raised crab apples. check out selection at Evergreen Gardenworks. All his crab apples, maples, and flowering quince are cutting grown, no grafted material.
Another tree to look into is Carpinus caroliniana - American Hornbeam, also called blue beech, and loose flowered hornbeam. They have wonderful yellow and orange fall color and are native to all of Wisconsin. So they will be fully winter hardy. Make great bonsai.
Related to hornbeam, Ostrya virginiana - Hop Flowered Hornbeam - also called ironwood. This is another Wisconsin Native - yellow leaves in autumn. Interesting seed "cones" that can hang all winter.
Elms - most elms do yellow, and only yellow in autumn. A choice one is Ulmus thomasii - rock elm, native to the sand counties of the driftless of Wisconsin. You might find it farther north. The American elm makes good bonsai also, can be found throughout the state. When grown in a bonsai pot the "Dutch Elm Disease" is not a problem, easy to treat if it does get infected. Siberian elm, a widespread invasive species is good for bonsai also. Keep in mind, Siberian elm absolutely needs full sun or it will drop branches on you.
Look around, lots of useful trees for bonsai growing in the Wisconsin north woods.