For every hardcore Marvel fan disappointed by Iron Man 3's complete refusal to look to the stars – especially following Marvel's the Avengers' mid-credits Thanos reveal – Thor: The Dark World's mid-credits scene seems designed to give the more cosmically-inclined crowd what they want.
Spoiler alert, but The Dark World's mid-credits scene places Sif and Volstagg in a setting we've never seen before: a corner of the Collector's tricked-out showroom. There, Benicio Del Toro – disguised completely in the Collector's Space Liberace persona – takes the ancient Aether out of the Asgardians' hands, thus keeping this Infinity Stone separate from the one already under Norse protection. This confirmed that the Tesseract is indeed an Infinity Stone, something previously revealed by Marvel Studios President Kevin Feige who previously outted the weapon as the space stone, and also confirmed that the phrase "Infinity Gem" has been left on the printed page. Now, they're "stones."
So with the Space Stone and the Aether, which we'll assume is the Power Stone due to its havoc-wreaking capabilities and color, accounted for, where could the other four Infinity Stones pop up as the Marvel Cinematic Universe continues its slow march towards the showdown with Thanos?
Soul - The most mystical stone of the bunch, the green Soul Stone can affect the souls of the living and the dead. The MCU has yet to venture into mystical territory, with the magic found in the Thor films explained away as an advanced form of science. But with a Doctor Strange film possibly on the horizon, it seems like the MCU is ready to take the plunge into the astral plane. If so, it makes perfect sense that Doctor Strange would come up against the Soul Stone.
Time - There's one franchise that manipulates time in a manner similar to the Time Stone, and it wouldn't be a stretch to believe that the orange gem could use next spring's Captain America: The Winter Soldier as an entryway into the Marvel Cinematic Universe. While we've already seen how Steve Rogers survived to the present day, how the film version of Bucky made it seventy years into the future mostly intact remains a mystery. It's possible that his nefarious employers could be using age-defying technology powered by the Time Stone, much in the same way the Red Skull used the Space Stone to power his weapons in Captain America: The First Avenger.
Mind - Because the two known stones in the MCU have both been used by villains, it would make sense to tie the blue Mind Stone to a villain known for mind control. It just so happens that Jessica Jones' arch-nemesis, the Purple Man, has that exact power. While Zebediah Killgrave gained his manipulation abilities via a standard chemical accident so often seen in comics, his power could easily be explained in Jessica Jones' upcoming Netflix series as a side effect of exposure to or possession of the Mind Stone. Nestling an all-powerful Infinity Stone into the main plot of Jessica Jones might also be a great way to get viewers to pay attention to a show starring one of the lesser-known Marvel heroes.
Reality - Next to the Power Stone, the yellow Reality Stone – which has the power to grant the user any imagined wish – has to be the most desired gem. The logline for 2014's Guardians of the Galaxy mentions that the titular ragtag gang of fugitives has in their possession "something everyone wants." Odds are that something is an Infinity Stone, and odds are it's the most powerful Infinity Stone. Considering how powerful this stone is, it would make sense for it to be one of the few – if not the only – stone to start out in the hands of hero rather than a villain. After all, if a villain has the Reality Stone, they're one wish away from dominating the universe.