I've always had a softspot for these, taking a more anime-ish approach rather than western media. More Gundam rather than either Star Trek or Wars. Giant capital ships, warmechs as heavy cavalry, space militaries, space battles, different planets, all the good stuff.

There are several different ways to go about the theme or how to start one. Including: 
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[*] A local capital ship is stopping in orbit of a planet to restock on a few different things, repair parts, fuel, maybe recruiting to replenish the ranks and train them while en route to the next battlefield.

[*] Already being a part of the ship's crew, maybe two or more members that are interacting for the first time, due to some movement in ranks, promotions or other shuffling around of personnel.

[*] After a battle of heavy casualties, two escape pods land relatively close to eachother on a nearby planets. Only trouble is, they are god knows how many miles from the nearest civilization and they are from the opposite sides, still, two people have a better chance of surviving together than just one.

[*] After a battle or even just spotting someone on a neutral planet, someone is taken as a prisoner of war to be interrogated on information that could help in the war effort, then go from there.
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Of course you are free to suggest your own ideas if you have any, always interested in something along these themes or even new ones. It's just the space sci-fi i like.