Og no! You want to hear the constant whine of their voices? Better to cut their tongues out.
Aldous Huxley suggested some:
I’d like to see us revert to the clothing of the 18th century France-the stuff worn at the Court of Versailles. A bit elaborate, but the time spent on dressing would prevent douchebags from being so…douchy.
But that stuff is so hard to keep clean. If you work outdoors, you are going to look like a pile of sticks and leaves in no time flat.
Footie pajamas, definitely. Or Snuggies.
Loose cotton drawstring trousers, loose cotton mandarin collared shirt (sleeve length up to individual). Canvas shoes, or maybe Crocs. Think chefs … or the Khmer Rouge. A chef/Mao jacket for formal occasions.
No, they can all wear jackets and jeans like the rest of us. But the hipsters would wear their tiny fedoras forward, and the douchebags would wear their plaid ball caps*** angled***.
My first thought for practicality’s sake was either a midriff-length sleeveless top and loose shorts for both genders, or something like fatigues, depending on weather. But that’s only a first thought.
There’s really nothing that’s appropriate for both Siberia and the Sahara with only minor alterations, let alone intergalactic travel. Could unity of colour scheme and design idea buy me some leeway with materials and layering?
Eww to the bomber jacket and jean pictures. The example for men looks pregnant and the example for women looks like an empty jacket. Make the jacket fit decently on either gender and you might have a deal.
I like the idea of the Mao suit, but I’m not so keen on the actual Mao suit in the pictures. Fit and form is the problem again, so maybe some real good belt or waist sash options would fix it.
ETA: When in doubt, togas. 
Velour baby!
Velour track suits!
I’m a right-winger, and I like Mao suits. I hate neckties.
When made of good materials, and properly tailored, they look good. See Blofeld and Drax in the James Bond movies. You’re right, a belt would improve the look.
I vote for ghillie suits. The aliens will have to find us first.
Robes and invisibility cloaks.
And WANDS!
No, NO, NO, a thousand times no!
That sort of thing drives me insane. I can barely handle a sport coat and tie for family weddings and funerals, and that’s for people I like - or I’m happy to see them gone.
My idea is a pair of comfy blue jeans, and either a T-shirt, or a sweatshirt. Colors and designs are up to the wearer, but mine must have pockets. Shoes can be tennis shoes or hiking boots, unless you’re doing something that requires specialized footwear - like skiing, or cycling shoes that clip into the pedals.
Oh, and one other thing - no capes!
Galaxy Quest black Jumpsuits!
I nominate the Shaolin monk’s getup. It would flatter a variety of shapes, and could be warm or breezy. Good color too.
http://www.chinapictures.org/images/shaolin-temple/1/shaolin-temple-kung-fu-monk-40203153924658.jpg
M.C. Hammer pants with a tunic on top. Black would be the formal wear.
Tassels and Speedos for everybody!!!
So, no-one going for the animal-skins-you-have-slain-yourself option. Disappointing.
Douchebags should be forced to wear a touch of mascara, and hipsters a giant gold sovereign ring and chunky gold bracelet. No, hang on, wait. They’d be happy to wear them ‘ironically’. Rethink required.
Standard NATO jumper and trousers, though I’m tempted to make those cargo pants instead for practicality (though I’m not sure how well those would go together).
Open necked shirt instead of with a goddamned tie.
Even have a couple modelling it!
http://www.bellworkwear.com/images/products/l_N200_1295283082.2103.jpg
Second choice is Ambivalids idea!