It's.a different ball game in a climate like California's. It is entirely possible to have a fall into winter garden here that is more spectacular in bloom and foliage in these seasons than in spring. I actually prefer a garden to be at its peak visually in fall, especially in November and December. Easy enough to do with subtropicals and high elevationtropical cloud forest plants from Mexico and South America.
In other less mild parts of the country with actual cold winters, the planting design would tend to follow your scenario. This is one of the reasons why I really enjoy designing gardens here in the San Francisco bay area!

I think the structure of plants is more important in the fall for several reasons. First, many of the other plants end up getting cut back sometime during the period during which fall plants are blooming or showing interest. Because there tend to be fewer plants in the garden, the structure of fall plants becomes more important and less easily camouflaged by other plants.

Secondly, part of planning for the fall garden is the use of plants that contribute to fall/winter interest as they are going dormant and one of the ways plants can do that is by having a strong structure. One way to enhance the structure of a fall garden is to use a mixed garden approach - shrubs and trees whether they are deciduous or not can add structure to the garden.

In terms of specific perennials, sedum 'autumn joy' is a good example of how strong structure can enhance the fall garden. Even when sedum is past bloom, it maintains its strong structure until it is knocked over by snow in my climate. An example of something that doesnt enhance a fall garden IMO, is aster that gets mildew. Its very difficult to hide the crappy aster foliage at the time its blooming. Certainly there are mildew resistant varieties that are a good choice.

Grasses are another good choice for structure and movement.

Another big thing about fall gardens around here is the visual competition. If you happen to have something that is flaming orange yellow, and sixty feet tall in your front yard, it's what people tend to look at instead of the little purple asters. So to a large extent, I've given up on small scale flower gardening in the fall because the real show is the fall leaves on trees and shrubs.

Following mad gallica's point... What do you mean by 'Fall'? Here, the trees and shrubs don't usually have a lot of fall color until mid October. So, to a large extent, I treat planning for fall flowers separately from planning for fall foliage color since they tend to have not a lot of overlap. The few things like 'White Pearl' bugbane that bloom in November when the main garden activity is raking leaves and getting things ready for their winter sleep are valuable but oddities.

I can't imagine my life without four seasons. However, where I live, autumn is much too short. Even if I were to plant the traditional fall flowers like asters and some sedums, they wouldn't thrive much before the first killing frost. The effort is just not worthy. Instead, I let the architecture of my garden preside, enhanced maybe with market pumpkins and big-box mums. Pedestrian, I know. But having said that...

Gardening is my enthusiasm, autumn is my favourite season. But my focus at that time of year shifts. I harvest the summer's yield, prepare the beds for the coming months and plant the bulbs for spring renewal. There is a different and restful beauty in the clean earth, the sculpture of the trees. My eyes turn away from the flowers that were at my feet to the sky, where waves of birds flow above flaming leafy canopies. I start to nestle, waiting for the first blanketing snow. I am humbled by Nature's own designs - I am just a provisional steward.

As you probably realize madtripper your list relies on flowers and colour so the other considerations are probably non plant elements. Unless you tune into the more European style that adds berries, seed heads and grasses (requiring space) you will have to consider shapes, longer shadows and what adrienne suggests so poetically.

Autumn here can be a quick explosion of intense color... late September into early October with the biggest show being the assorted maples. I don't plan or plant for it because the "brown season" is generally the few days between the first killing frost and the following measurable season lasting snowfall.

An odd, late starting season - plenty of rain at regular intervals, but cold until mid-July. Perennials pretty much on time, but the annuals languished for a long while.

Working like a crazy person, all the gardens got weeded, some perennials got split or relocated or consigned to the compost pile. For a few short weeks now I get to walk around and survey my realm without really having to do anything. But I rather enjoy it all and will not repeat last season's mistake of not doing a thorough fall clean-up. After that, I'll be content with the conifers and bare branch patterns and the sound deadening silence only a deep snow cover can provide.

Autumn can be quite short or prolonged depending where one lives. Our springs in the PacNW are quite rainy and the joke is that summer doesn't begin until July 5. This year we really haven't had much of a hot summer at all. Our warmest months are August, September, moving into October with an Indian summer.

I would think that structure would be very important since that is what one will be seeing after the perennials die back.

I lean towards interesting conifers and evergreens,perhaps with variegation. I am drawn to the blue and the gold forms. After conifers, deciduous trees/shrubs with interesting branching patterns or color such as the yellow or red twig dogwoods. And of course anything with berries.

I used to live in the San Fran Bay area and loved to see the Pyracantha and Cotoneaster plantings along side the roads in the fall.

Consider height variations, and by that I mean topographically. Create a raised bed that complements your existing bed and intersperse your fall interest plants with the other season plants using the height gained by your raised bed to your advantage. Also, think about using plants that maintain their structure all through the season and are not JUST a focal point in the fall. Look through magazines or plant books for plants that star in the most seasons possible, or have the longest blooming period. This way, your garden's transition from spring to fall is not a dramatic twist through every season but shifts naturally and gracefully.

Being in the Midwest, we have lovely, gorgeous autumns most years, and the trees and shrubs are really the stars of the show. For me, the important thing to realize was that anything I want in bloom in the fall needs to work with the changing leaves. Pale pink is not a good fall flower color, imho! (I do usually have some cosmos still blooming at that point, and it looks a little weird.)

I do have a number of plants still blooming into autumn, most of which stay for winter interest - white boltonia, agastache, sedum 'Autumn Joy', and Russian sage. I haven't planted any grasses, but they are another classic autumn feature that I think works beautifully with autumn colors.

(One thing I did last fall was plant bulbs between my boltonia and agastache, to have some spring interest there. The only problem was that the boltonia and agastache were so huge at that point, it was difficult to get in between them to plant the bulbs!)

Another thing to take into consideration is whether you have any intentions of doing anything special for Hallowe'en. ;)

What is fall? I think Fall starts around mid Sept in zone 5. By then most summer flowers are finished.

A number of the comments are design considerations for fall, but they also apply to the rest of the year eg consider the structure of plants.

I was looking for design criteria that would apply to fall and much less so to spring and summer.

Hiding spring and summer dying plants is more important for the fall design. The change in color foucus towards mostly reds and oranges would also be a consideration for fall.

Not to be difficult, but your OP mentioned fall blooms. Now you say that most summer flowers are done by then. It all depends on what you have blooming in your garden, the weather that year, etc. I've had plenty of flowers blooming in September, even through November, in both of my gardens. These include annuals from seed (esp cosmos and zinnias), or perennials such as hollyhocks, which in a good year start blooming mid-summer and go through the end of November. Then there are also the flowers that don't start blooming until September, like asters or the deepening colors of my sedum Autumn Joy.

Playing along with the theme of more traditional fall accents and how best to feature them for maximum impact,several thoughts come to mind. First, fall foliage color shows best against darker greens of evergreen or coniferous trees, or buildings. Fall foliage will tend to last longest if trees are sited to protect them from strong winds. This is particularly useful to remember with Gingko trees here in northern California. Drifts of shrubs with fall foliage color have greater impact than single accents, and more pleasing still is to combine variations on a color theme. We can often get very good fall foliage color here in coastal California with the right choices, and summer drought stress can even give early fall color on trees as early as August.

Fall flowers can just as easily be vivid saturated tones here in a Mediterranean climate such as California's. Some of my favorites include vivid satiny fluorescent pinks of Nerine bowdenii and. Schizostylis coccinea or softer pinks of towering 20 foot tall Dahlia imperialism or chrysanthemum -like Tree Daisies such as Montanoa grandiflora which also smell like freshly baked sugar cookies in fall bloom. As well, there are almost too many fall flowering Cloudforest Salvias species for prominent end of the year bloom here, in jewel tones of deep blues, burgundy, rose pink, lavenders, and yellows. Salvias such as S. wagneriana, involucrata, splendens 'vanhoutei' to name just a few.

I personally find it more fun to plan/design for massive fall blooms mixed with more traditional fall foliage than a big spring bling. In fact, the start of the fall rainy season here locally really is our equivalent of new growth and the native flora waking up again after 6 months of no rain. So really, October is our true spring as it heralds the hillsides turning green again, and the first flowers of natives such as Ribes sanguineum come as early as November.