One of the problems with Protestantism is the tendency to division, but not because of heresy (in the sense of the "revelation" of some other truth). Rather, it is because each person takes a splinter of the whole truth and sets sail upon the sea of life astride his own little splinter. At first, other people will get on the splinter, and then suddenly, one of them finds a splinter of the splinter that appeals to him even more, and he sees that as the entire truth and sets forth on his own little splinter-borne journey. And so forth.
The Protestants didn't really take anything away from the Church, although their overemphasis of their particular splinters of the truth led to a defensive reaction that resulted in closing the mind of the Church to many things that were actually in the process of being worked out.
What Protestants always overlook is that the development of doctrine is a long and complicated process, and the person who is unfortunate enough to have to preach the truth can expect to suffer for it. Many of the people the Church reveres as saints today were, in their time, condemned and even excommunicated. Even Athanasius found himself fighting for the truth alone. But they remained part of the Church, patiently speaking the truth as the Holy Spirit revealed it to them in accordance with tradition, obediently accepting correction and even silence, because they did not have the arrogance to think that their splinter of the truth was the only one.
They were also confident in the truth, confident in God's power to reveal the truth, and confident that if they were right and what they taught came from God, the Church would accept it. They were absolutely right in this, and instead of their truth becoming a separate truth or shard of the truth, it became part of the Church's deposit of truth shared among all believers.
To me, a heresy would be something like Islam, which takes a few scraps of Christian imagery, a few scraps of Jewish ritual law, and a few bits of paganism and creates an entirely new and awful belief system.
Protestants do not do that; I would say that what Protestantism does is take a truth, and emphasize that truth to the extent that it becomes much bigger in the scale of Christian belief than it should be. And then as a reaction to this overemphasis, another Protestant might take an opposite approach (I'm thinking of Protestant struggles over grace and free will, for example), which then will lead to another division and another attempt to constitute a partial truth as the foundation.
I do hereby conclude: When the Western Church fissiparated in the sixteen century, the Reformers took a portion of the essential patrimony of the Church with them, and they thereby left both the Roman Church and themselves the poorer for it.
I have been coming to the conclusion that the Latin Church views heresy somewhat differently than Orthodoxy does. By your definition, Arianism, Nestorianism, Sabellianism, Montanism, Donatism, etc. etc. would not have qualified as heresies yet the Fathers and the councils condemned and anathemized them as heretics. When you look at their theology (except maybe the Arians and the Montanists in my list)and even more so their praxis (except perhaps the Montanists)what you see is in most senses the orthopraxis of The Church.
When we in Orthodoxy look at "Protestantism" we see, even in its wide diversity, very little which looks like the Faith of The Church except in some basic Trinitarian and Christologic theology. We see virtually nothing of the Mysteria or liturgical praxis; we see virtually nothing of Holy Tradition. Its sotierology is almost completely different from ours. To us that spells heresy and heretics.
On the other hand, it is interesting and I think informative to remember that Arians, for example, were received into The Church by a simple profession of Faith, sometimes with and sometimes without chrismation which speaks volumes about what The Church thought of Arian baptism, for example.
Sitting in the East and observing the confusion of Western Christianity, we have to conclude that when the Protestants did leave the Latin Church, they did indeed take a great deal with them including a sort of proto resistence to the pyramidal ecclesiology and the marginalization of the laity and lower clergy from the workings of The Church in the West. To tell you the truth, I think that in many senses the Protestants have a better intuitive understanding of the fullness of The Church, or better put, where that fullness is found, than the Latin Church does, even if what they have created don't actually qualify as churches because of the truly astonishing amount of the Truth of The Church they rejected and left behind.