I like the theories shared here so far.
However, I do think that
Spoilers
Loki might come back, because due to Ragnarok and the character’s success in it, they might decide to resurrect him for (if they end up doing) Thor 4. I hope Vision and Gamora are gone for good though. But most theories here regarding them sound much more convincing than actual death for the characters.
More Spoilage
Nah, Loki needs to stay dead at this point. Bringing him back would be so very cheap.