I would suggest Franklinea, but I just got a new "bible" and Michael Dirr says these are too unreliable; specifically he says gardeners are "better off not falling in love with them" - or something very similar to that.

I also really like the witch hazel, Hamamelis x intermedia 'Arnold Promise' because it blooms in February, with bright yellow flowers that often last for well over a month - maybe two, depending on the weather. It is longer-blooming and more tree-like than some other witch hazels; mine's about 10' tall now.

Cornus mas is nice, it has tiny yellow flowers in very early spring - after Arnold is done. It may prefer more shade than you've got, though. Mine was shaded by a nearby tree that we recently removed, and its leaves look a little sunburnt this year; it's a variety with pale leaves to begin with.

I don't grow prunus mume, but it looks like one of the nicest flowering cherries. I inherited kwanzan cherries and do not recommend them - they're awful and I"m looking forward to replacing them with something nicer... some day.

Styrax japonicus is a great tree, but seems to want to get as wide as it is tall. Since its arching form is really lovely, it wouldn't be useful if it were pruned to fit into a narrow space.

There are some great dogwoods - I wouldn't plant the native Cornus florida, because it's so susceptible to disease, but C. kousa is lovely and grows fairly fast.

Just how much height are you looking for? Will it be to the bottom of the eaves, or can it grow up to the roofline? Do you know the height and width you want, or are you just thinking "not maple/oak/beech sized"?

For a spring bloom I'd look for Cornus kousa (smaller cultivars).
For the mid-season- Stewartia sinensis.
For the late bloom you may consider either Crape myrtle (z6 hardy varieties) or Heptacodium miconoides. Both could be grown as a large shrub or single trunk tree.
All of the above are potentialy 12-15' tall trees.

Heptacodium CAN do a disappearing act, caused by some (as yet) unknown canker-type blight, according to my new copy of Dirr, so I wouldn't recommend it for an anniversary planting or for a new gardener. I've only had mine for a couple of weeks, so I can't claim any direct experience with it. (except that I haven't killed it yet!)

both cornus kousa and jap maples will give you 3 seasons of interest. drive around your town and see examples others have used.
best,
mindy

Amelanchier - Serviceberry.

Stewartia Koreana
Nice shape, beautiful flowers, nice fall color.

How about a flowering crabapple? There are cultivars that don't grow too large and are disease-resistant. In spring they are loaded with wonderful fragrant flowers that attract butterflies, hummingbirds etc. and right now the tiny berries are changing from yellow to a lovely red. Birds will be all over this when the berries are a little more mature.

We always had crabapples but they were always ratty looking and attracted pests and leaf disease, and the fallen apples were a mess. The cultivars now available seem to solve those issues. Highly recommended!

Another possibility - our native fringe tree (Chionanthus virginicus). A small spring flowering tree, covered with white "fringe" flowers, 12'-20' tall. Here's a picture:

http://www.mobot.org/gardeninghelp/plantfinder/Plant.asp?code=C120

I just bought a three foot tall Heptacodium miconoides for the corner of the house between the front and side porches. On sale for $24. Read about the canker but there seems to be uncertainly over the degree of the problem. A nursey by me (which has a 6 year old 25 foot tall heptacodim planted in partial shade) told me they only had a few large specimens (at $700 apiece) and sold out this year by June. The horticulturalist at the nursey told me she thinks hers grew so well because she planted it in a well watered area. Considering the desirablity of the tree and its four seaons interest, I figured it was worth a shot. Plan on taking cuttings in the spring and putting in a half dozen of them.