Now we know that in the MCU the stones are not gems but objects of great power, that can be any shape or consitency, even objects that in the 616 universe are a completely different entity.
We already encountered two of them, the Tesseract and the Aether. Personally I find it brillant, that they dangled the first gem right before our noses throughout Phase 1. Reimagining the Cosmic Cube, which is an artefact of enormous power, as one of the stones was something that wasn't speculated by anyone here on this board or elewhere and came with a surprise and it makes me wonder about the nature of the other stones. will they be objects, we already know in some form or the other from 616 or will they be completely made up like the Aether? My money is on a good mixture and I guess every new McGuffin introduced in the next 1 1/2 Phases is suspicious from now on. Shape or size don't matter any more.
I guess GotG introduces the next stone, after all, this movie has the closest ties to Thanos and the collector and the new banner even features a strange new object prominently
And after that? I guess a Dr. Strange movie is the right place to introduce number 4, but I can't see one of them appearing in Ant-Man or A:AoU or in any of the cap movies. That leaves us with Thor 3 for Gem 5 and a possible GotG 2 or A3 with Gem 6. but this would make Phase 3 kinda repetitive with a lot of franchises depending on a powerful McGuffin that will be to overcome or gathered. Maybe they stretch it out to Phase 4 and Thanos won't be the supposed villain of A3, but saved for later?
Another thought. In 616 every stone controls a very specific force. I didn't get the impression both the Tesseract and the Aether were similar to that depiction. both were incredible powerful objects but not defined by a force as it seemed. Or were they? It was often believed that Loki was using the mind gem in TA. could that still be true? his staff was powered by the Tesseract, would that make the Tesseract the mind gem? if so, maybe we have to rethink all ARC reactor thories or is Tony Starks suit mind powered now?
If the Tesseract is the mind gem, what about the Aether?