1. Suspenders
Or braces, if you will. And I don't mean the cheesy clip-on suspenders either, though those did exist back in the day. I also don't mean boring black or tan button suspenders (though they have their place when the right occasion calls for them).
I'm talking about colorful button suspenders we never see today apart from the occasional eccentric dandy. Like many of the other items on my list, these bright pant-holders appear too 'costume-ish' to the modern eye.

2. Walking Stick
Not your grandpa's cane nor that of the inner city pimp. A bamboo walking stick is great for walking downtown on a warm summer night, just as long as you are dressed accordingly. But again, society has rejected this once common article as something for the snob with his nose high in the air.
Of course, a walking stick with a sword is preferred since a gentleman needs a stylish form of personal protection. Also comes in handy if you feel like breaking into a dance number.

3. The Boater
Also known as the "42nd Street Skimmer". A difficult hat to pull off, it was a summer favorite of the common man as well as the FBI for a short time. Long stares and open mouths follow this hat wherever it is worn, so be prepared for the fanfare.
And while Jeeves disapprovingly points out that "gentlemen do not wear straw hats in the metropolis, sir", at times I can find him a bit stuffy. Wear it wherever you wish in the warm months.
Need I say more?
Or as our English friends like to refer to it, the Cravat. This article, above all else, is considered today to be the sure sign of a foolhardy snob with more money than brains. However, this casual look was popular with men of the middle and high classes back in vintage times, especially in the 1930s. While still a necktie, the loose and flowing ascot provided a flowing cascade of cloth that added a dash of adventurism to any safari jacket or partially open button-down shirt.
A favorite of Fred Astaire who often wore it with a scarf tied around his waist as a belt. Now that is style.
Let's hear what obscure and now scorned items you would like to see make a return.