Doctor Who frequently brings the alien down to Earth. Often literally, in the TARDIS, with a thump. Even when visiting other worlds, or space stations or Moonbases or what have you, the travellers mostly stay indoors or in quarries. But now and then it goes space opera and gets rather Star Wars, or a bit Dan Dare, or a lot 2000AD, and we find our heroes flying through wormholes as well as running through corridors. Big sensawunda SF, multiple alien types, spaceships swooping around, the whole thing.
4: One condition, it has to be amazing!
5: Get to the escape pods!
This didn't get into a series-ful of plots series one because it's fairly rare on the show, less so now than back in the day thanks to bigger budgets and cheaper effects but still no more than one a series. It's more the territory of the spinoff novels, audios and especially comics - the first issue of Doctor Who Weekly started with a Pat Mills and John Wagner story illustrated by Dave Gibbons in which the Fourth Doctor fends off an invasion by a parallel futuristic Roman Empire with armies of robot soldiers and alien gladiators.
Space opera is by and large fairly straightforward - big spaceships, swarms of little spaceships, weird aliens, laser gunfights, exploding planets... Doctor Who sketches in a cosmopolitan universe of many alien species which is happening right now, and a future for humanity that includes space opera as well as horror-SF among the genres it overlaps. Earth can be endangered by alien armadas, or a future human colony can be a fairly major setting, something to get a number of humans and humanoids on screen along with the aliens and robots and all, but even this isn't essential as long as the travellers have someone to talk to and the audience have people to worry about.
For Gamemasters, a space opera adventure has one obvious advantage - you can grab an adventure from a suitable RPG and drop the TARDIS into it. Plenty of Traveller, Star Wars and Star Trek adventures just need a new motivation to bring the PCs in, renaming of canon elements, and a scaling down of expected PC firepower, and you're good to go. This also applies to suitable TV episodes, books and comics. (Or even theoretical comics like I Should Write The Legion.)
Example: Armies Of The Night / Death In The Stars
The Third Great And Bountiful Human Empire plunges into civil war. Ships loyal to Emperor Solkan and Archduke Morlen ravage systems and planets indiscriminately, both sides hunting for the ancient lost technology that will let them win the war decisively.
The Doctor, Emily and Richard find themselves targets of both sides, hunted by Raptor and Thunderbolt fighter ships in space and hovertank and swoopbike hunters on the ground as they shield a refugee, a child with the genecode to unlock the power of the ancients...
(The power of the ancients could be just about anything - time travel, a caged black hole, a legion of sleeping Daleks...)
Can they win this battle and turn the galaxy away from a war that will destroy humanity and countless other species?