I somehow think that the enchantress will return for the third Thor movie. I hope for her and Loki to free Surtur. I donīt really see Loki in AoU so he better play a big part in the third Thor movie because I donīt think his role in the second will be as big as some ppl think.
Okey I will try to approach it from your angel
Lets play with the thought that Ultron actually got a powergem from someone like the enchantress.
First of all, I know you didnīt mean it like the red light comming out from Ultron is to big of a coincidence because the fact that the powergem is the gem Ultron actually could use (unlimted power = dream for a mobile robot). You meant it more like something you can add to a pile of arguments for why itīs possible. But I wouldnīt add that to the pile because Ultron has a red light even without the gem. So thatīs why you got that response.
So someone from Thor like the enchantress takes the IG from Odinīs vault (or picks it up somewhere else). Why would she give it to Ultron?