Some things I would like to point out, regarding Wanda and Vision:
Wanda is a human becoming a God-like being.
Vision is a God-like being becoming more human.
Thus, two souls on opposite paths that intersect.
At the end of Age of Ultron, Vision states that Order and Chaos are not, contrary to what humans think, opposites. Vision represents Order, Wanda represents Chaos.
Vision is powered by the Mind Gem. Wanda is not. Consider Pietro's powers, and Wanda being vastly different from Vision. Wanda's powers are connected to her emotions, wheres Vision's are not. She is, however, connected to the mind gem.
Chaos Magic is connected to emotions, and was created by the Elder God Chthon, also known by his alias, The Other.
The scepter was given to Loki by a being called The Other.
Add to that, Wanda somehow survived a Stark bomb and Hydra experiments, the odds of that are staggeringly long.
To me all the above points to hidden, latent powers which the scepter/mind gem, unlocked, with the tk and telepathy being by-products of mental augmentation. She's powered by Chaos Magic, but only uses tk because she does not know how to use magic yet.
"I control Chaos the way others control clay."
- Wanda Maximoff
We have pondered Thanos vs the Avengers/Guardians, and not considered the possibility of a power play, an opportunistic trick by a third party.
At some point there has to be a twist, a curve-ball.
I believe if there is going to be a wild card in Infinity War, it will be Chthon. He will want Wanda to fight Thanos. He wants Vision to die. With all the stones gathered in one place, with Wanda being pushed to extremes, his goal may be for her to destroy the stones in an explosion of rage. Thus, the elements of order are banished from the universe as it implodes and rebirths, and Wanda is utterly corrupted; there is nothing the Elder God loves more than destroying good people.
Thus Wanda, in those crucial moments, decides the fate of the universe...and the choice is a hellish, Catch 22/Kobyashi Maru choice between victory for Chthon or Thanos. The latter being the good option.
A central theme of Wanda's arc is control, and the death of Pietro foreshadows the end of A: Infinity War; intense grief and pain triggering a loss of control, and a loss of reason. Her growth as a person will result in her not giving in to her pain and anger.
She defeats the demon within her, but in doing so has to hand Thanos victory.
Strange then teleports Wanda to safety, doing so so as to trick Thanos into thinking she, one of the few beings who can challenge him, is dead, thus letting the Mad Titan think he has won.
Wanda's connection to the Infinity Stone sustains Vision within the Infinity Gauntlet.
"Perhaps I, too, shall be saved by love"
- Vision
At the start of Infinity War, Vision is almost certainly at Avengers HQ.
Wanda could be in all sorts of places:
1. With Team Cap
2. or if team has splintered as they hide, with Clint.
3. With Strange (though how could she, a fugitive, live in New York?)
4. With the Ancient One, at her temple.
5. With Agatha Harkness in the MCU version of New Salem.