No matter how virulent and radical the reformer, it seemed nobody in the 16th and 17th centuries wanted to be labelled a Pelagian or a semi-Pelagian (In fact, despite criticism of many Catholic teachings, I think few were willing to deviate from quasi-gnostic/manichean Augustine and the early Church architects, at least none I can note off the top of my head).
My question is, is it possible for a major (as in, as major as Calvinism or Lutheranism) Protestant denomination to arise that officially follows Pelagian or semi-Pelagian doctrines, and how would that come about?
As you said, Pelagianism was a
label. In Middle Ages, one could be labelled Adoptioanist or Manichean, without being actually so but because it looked like (sometimes remotly) how these ancient heresies or beliefs were described.
Similarily, Semi-Pelagianism/"Remains of Pelagianism" (as St. Augustine called it), is a name applied to a range of beliefs, not the name used by people that generally saw themselves as an orthodox believers within the Church.
Eventually, as the name became strongly associated with heterodoxy, with all the negativity it carried, I doubt one would have claimed being so.
Now, what about a trend that would look like Pelagianism, without being called so?
Maybe, but then, Protestant churches are likely to not abide by : Calvinists especially called Pelagianists people that disagreed with the concept of predestination.
You'd have better chances with Catholicising beliefs, *maybe* with a deeper split with, for exemple, a semi-Jansenism being largely widespread among XVIIth Catholics, forcing a belief (semisemiPelagianism?) to arise among Catholics and to give a different vibe of Old Catholic teaching?
Not the most likely outcome or TL, but I can't really think of much else.