Not all loadouts here are sci-fi themed because some classes just don't have any good futuristic cosmetics. This mainly includes soldier but a few other classes have like 1 or 2 good ones.

Links lead to backpack.tf pages, where you can buy cosmetics from trade bots.

If you want to know what a slightly altered loadout here would look like, you can try http://loadout.tf. It's very easy to use.

I used to use Cadet Visor but I swapped it out for Bolt Boy because cadet is invisible for some reason. I think bolt boy looks better anyway.

Fuel Injector[backpack.tf]
Bolt Boy[backpack.tf]
Flak Jack[backpack.tf]

Scattergun skin in screenshot is Geometrical Teams.

As i said in the intro, this class lacks sci-fi hats so I went for the militaristic aesthetic instead.

Coldfront Curbstompers[backpack.tf]
Flakcatcher[backpack.tf]
Killer's Kit[backpack.tf]

Paint the Crusader's Getup whatever you want, I use green but some others like pink or team-colored may work well.

HazMat Headcase[backpack.tf]
Crusader's Getup[backpack.tf]
Pyrobotics Pack[backpack.tf]

Eyeborg[backpack.tf]
Juggernaut Jacket[backpack.tf]
Double Dog Dare Demo Pants[backpack.tf]

U-clank-a[backpack.tf]
Immobile Suit[backpack.tf]
Purity Fist[backpack.tf]

Sightliner[backpack.tf]
Iron Lung[backpack.tf]
Roboot[backpack.tf]

Wrench skin in screenshot is Hazard Warning.

Flatliner[backpack.tf]
Kriegsmaschine-9000[backpack.tf]
Herzensbrecher[backpack.tf]

Letch's LED[backpack.tf]
Snow Scooper[backpack.tf]
Triggerman's Tacticals[backpack.tf]

Update 29/7/2020: I reworked my spy loadout because I was least satisfied with it. Soldier is now my only non-sci-fi themed loadout.

The Nanobalaclava[backpack.tf]
Lurking Legionnaire[backpack.tf]
Sky Captain[backpack.tf]