4. Just Who Is Krasko?
"Rosa" is a tremendously strong episode, and yet it does have its weaknesses. The villain, Krasko, is pretty one-note. He's a white supremacist from the 52nd century, apparently responsible for an atrocity that killed some 2,000 people. In truth, Krasko is really just a plot device to force the time-travelers to embroil themselves in 1955 Montgomery. That means his character and motives aren't exactly explored in detail - they're simply not relevant to the plot.
What's interesting, though, is that Krasko is possibly the first Doctor Who villain since the Meddling Monk in the William Hartnell era to truly see the potential of time-travel. There's a sense in which he's one of the smartest foes the Doctor has ever fought - a villain who realized he could change history by just making small adjustments to the timeline. It's implied that he's a rogue Time Agent, a man who had previously policed time travel but has now decided to abuse it. According to one of the Doctor Who novels (which may or may not be canon), Sometime Never, "The Agents' purpose is to change without interfering, to leave an effect with no evidence of cause. To be untraceable, undetectable, invisible. To all intents and purposes, the Agents do not exist." Krasko is following a similar modus operandii, attempting to change history while remaining as invisible as possible. Unfortunately for him, his vortex manipulator leaves traces of Artron energy, and thus his very presence in 1955 drew the Doctor's attention.
3. Is Krasko Still A Threat?
Ryan is the one who defeats Krasko, using the villain's own weapon to send him back in time - to an unspecified date in the past. The fact that Krasko is still technically alive, lost somewhere in the vastness of history, means that he could still be a threat.
That seems unlikely, though; Krasko's temporal displacement gun was designed to send people several millennia into the past or future, so he's probably been blasted back to a time before America was inhabited. What's more, he also doesn't have any of his weapons, and the Doctor had already destroyed his vortex manipulator. It's more likely that he's a "Villain of the Week."
2. Just How Common Is Time-Travel Technology In The Future?
We've long known that humanity would develop time-travel technology in the future. The Time Agents were first mentioned in the Tom Baker era, after all, and Magnus Greel was a human warlord who sought sanctuary in 19th century England in "The Talons of Weng-Chiang." But the fact a former convict of Stormcage Prison could get his hands on a vortex manipulator is telling. It suggests that time-travel technology may have leaked on to the black market by the 52nd century. His temporal displacement gun, too, is a fascinating weapon; anything hit by the beam is displaced in time, irrespective of the potential damage it causes to the timeline.
No wonder humanity formed the Time Agents to police time and try to keep history on track. Frankly, it's possible "Rosa" actually saw the Doctor pick up the kind of mission that would typically be handled by the Time Agents.
1. How Did Krasko Know About TARDISes?
Tied to this is the fact that Krasko recognized the TARDIS for what it was. Not only has he acquired Time Agent technology, he's also apparently aware of the Time Lords. That said, his comment about the TARDIS's value strongly suggests he views the vessels as antiques that have been picked up by collectors, and had no idea he was dealing with lan actual Time Lord. It's possible his knowledge about the TARDIS is one reason the Doctor avoided introducing herself; she may have been wary of just how much he knew about her.