Re: Wanda or Vision, who should turn up first?
I hate to say this, but I'd be perfectly okay if the Avengers introduced Wanda to the MCU, but left Pietro out. Wanda is crucial to the Avengers, but Quicksilver was just always....sorta there. Let's face it, he's always been a poor man's Flash, and he's never been integral to any major story arc and definitely doesn't have a huge fanbase or even fan recognition.
With the advent of Dr. Strange and full-blown magic, Wanda's story can be upgraded easily enough to become a "true" Scarlet Witch, and that would be a simple enough concept for general audiences to swallow. A speedster is a much harder origin story to develop in the MCU without the benefit of super-mutation; short of an Eastern European nation super-steroiding the hell out of an athlete to cheat their way to Olympic gold or something, I don't see a good fit for Pietro.
THE COTTON AVENGERS
...They move like slick cotton on oil.