If you aren't up to date on your Marvel movies, you probably don't want to read this article. If you have, but haven't seen Cap 2 yet then you should go see it, but this won't spoil any of that.
As soon as Thanos showed his scalloped chin at the end of Avengers, any comic book fan worth his salt saw the Infinity Gauntlet coming (especially since it was actually shown in Thor).
It soon became apparent that the portal-controlling Tesseract was indeed the Space Gem...
And we were quickly introduced to the mind-controlling Mind Gem contained in Loki's scepter...
These were still speculative until the after credits scene from Thor: The Dark World where Sif tells the Collector that they didn't think it wise to hold two gems in the same place, leaving the Aether with him. The Aether was first assumed to be the Power Gem, but now I believe it to be the Reality Gem (more on that in a second)
Feige has recently revealed that the Orb from Guardians of the Galaxy will instead be the Power Gem, which is what makes the Aether into Reality. (An interesting side note is that Scarlet Witch's reality bending powers have a similar red glow)
So we still have to be introduced to the Time Gem, and Soul Gem. This is where I see Marvel having a couple of viable options. We know that a Dr Strange movie is in the works, and they could choose to use the Eye of Agamotto to represent the Soul Gem.
The only issue I have with this solution is that the above picture clearly shows it in Odin's Vault, meaning that to keep the Aether in Asgard would have kept THREE stones in the same location. I'm not sure how they will fit it into the "Phasing", but I'm pretty sure we've seen Adam Warlock's cocoon in the same scene with the Aether. Is there a chance that Adam is already in possession of the Soul Gem, or is actually a physical embodiment of it, unbeknownst to the Collector?
Whichever route they go (or if they have another surprise up their sleeve), clues for the Soul Gem already exist. I'm more excited about the idea of the Time Gem. If the Infinity Saga isn't actually slated for Avengers 3, then we could easily see Kang bring the Time Gem with him in that installment.
The other option is to have Thanos get his hands on four or five and have them lead him to the last.
Whatever they decide, I am definitely excited to see the continued unveiling of these stones. I would really like to see Loki (Now king of Asgard) develop a cabal, the Masters of Evil. They could find Red Skull, the REAL Mandarin, and Parker Robbins (assuming he is the villain of Dr Strange) and have some henchmen below them. That way they would help in combating Thanos since if he wins, there is no place for them to rule.