I don't know whether this has been written before, probably has been, but I couldn't resist. I'm a little in love with the idea.
Spoilers for The Big Bang: set during that episode, kind of, as Rory waits two thousand years.
Captain Jack Harkness meets Rory the Roman!

Picture the sight: a huge warehouse, aflame, fire shooting out of the doorway. And in the doorway, a huge cube, completely black, with circular markings on each side. A figure strains to pull this cube to safety; a Roman, clad in armour, a plumed helmet, even a crimson cape.

The Lone Centurion struggled with the Pandorica; his burden to bear for all these centuries. But he did not mind; all his time, his mind, his body, his heart: it all went towards the box.

World War Two. He'd narrowly avoided a bombing; and now tied ropes around the perfect prison, and the perfect sanctuary, straining to pull out the Pandorica. Too heavy; for once, he was struggling, heat of the flames making his plastic arms weaker. But he kept trying.

"You look like you need some help," a distinctly cocky voice sounded; and the Centurion felt his load ease; straining to turn, neck moving with a great deal of ease, partially melted, the Lone Centurion saw the base of a soldier's jacket behind the Pandorica; a man pushing the cube outwards.

The Lone Centurion sighed in relief: out of the flames, and the burning warehouse. She was safe, at last.

"I've been saying you around," the man ran around from the back of the Pandorica. He had short, black hair and a long, flowing jacket. "Captain Jack Harkness," he extended a hand, one the Lone Centurion took, hesitant. "And who are you?"

"I'm-" the Roman cleared his throat. It had been decades since he'd last spoken. "Rory. Rory Williams."

"Very Roman name," Jack grinned, appreciatively; before his face became serious. "Now do you mind telling me how a regular human's been up and running for centuries? I've seen you in the history books."

Rory met the Captain's eyes. Jack found himself in awe; there was such wisdom in those orbs. Age, knowledge, too many sights seen, and yet the Centurion regretted nothing.

"I'm not a 'regular human'," Rory spoke, voice still hoarse, "Plastic," he lifted his sword-hand, blade in its sheath. Click. The fingers slid away; the Auton weapon was revealed for a few seconds in the firelight.

Then, ignoring Jack's stare, the Lone Centurion fixed his hand, lowering the arm. A longing look back at the Pandorica.

"Plastic, right," Jack said, hesitant; he wasn't shaken, as such, simply surprised. His experience with such duplicates hadn't prepared him for such a, well, human encounter. A grin; "Well, that can be fun."

Rory was silent. The Roman removed his helmet, holding it beneath his arm, and sat on a piece of wreckage, feeling his skin start to dry. He looked solemnly at the Pandorica.

"Why're you lugging that huge box around?" Captain Jack spoke, gesturing towards the cube.

"I," Rory Williams paused, clearing his throat again. "A promise I made to a friend. I'm not going to let her down now."

Jack Harkness moved quickly after that. It was something about the devotion in the Centurion's eyes. How could anyone who felt such passion be no more than plastic?

Before the Roman could react, Jack took a step closer, lifted Rory's chin with two fingers, and planted a light, but meaningful, kiss on his dry lips.

A pause; silence. Jack and Rory watched each others' eyes. Slowly, Jack lowered his hand from the Lone Centurion's face.

"I guess I shouldn't have done that," Jack murmured eventually.

"No," Rory exhaled; strangely, he felt like smiling. "My fiancée's in there," he gestured towards the Pandorica.

Captain Jack blinked, staring at the Roman.

"They're always married," he muttered, quietly, to himself. Then: "You keep your fiancée in a box?"

Strangely, yet again, Rory found himself chuckling. There was something oddly likable about Jack.

"It's a long story," the Roman said, getting to his feet. Another, solemn look towards the Pandorica.

"Any idea how long the lucky girl's going to be all boxed up?" Jack asked, somehow still grinning broadly, taking a step back as he watched the Lone Centurion pick up the ropes once more.

"Sixty years," Rory replied, hefting the harness onto his shoulders, taking a few steps; the flames had dimmed now, giving his plastic flesh a chance to set. Stronger. "At least, that's what the Doctor said." The Roman began to pull.

At those words, Jack span around to face the stranger; his jacket whirled upwards in the wind. The Doctor?

Yet as Captain Jack Harkness looked, the Lone Centurion and the mysterious Pandorica had gone. Once more in the realms of myth.