It appears that the Council of Nicaea was a well planned and well executed affair. When Constantine ordered it be created, did he have any historical predecessor to examine how he should do it? Were the structure of the meetings based on any other debates previously held in Christianity or another religion?
Drawing particularly on Hamilton Hess, The early development of Canon Law and the Council of Serdica, especially chapters 1-3, prior to Nicaea there were various types of church councils, primarily local in nature, and in the 2nd century and 3rd up to circa 250 AD they appear to be more deliberative assemblies. In the second half of the 3rd century they begin to take on more legislative and judicial functions, adopting procedural elements modeled from Roman governance practices. By the early 4th century there was thus a church precedent for councils as deliberative forums that would follow patterns of Roman governance. At the same time there certainly wasn’t a sense of a ‘whole-church’ council that would deliberate and make authoritative pronouncements for the whole church. But Constantine would certainly have been familiar with the various levels of Roman government and their conciliar and deliberative procedures, and even early on in his career as Emperor moved to establish Christianity as both a unifying and unified force in the empire. This can be seen in his earlier moves against the Donatists (the edict of 317 against them, overturned in 321 by granting toleration).
I would suggest it’s against this backdrop of church councils that Constantine decides to call Nicaea, as an attempt to deal with what, by that stage, was beginning to be more than a local issue, as both Alexander (of Alexandria) and Arius had acted to broaden the scope of their dispute by involving other sees and bishops on both sides. Also Constantine was no doubt guided by Ossius of Cordoba in formulating the idea of the council. That a sizable amount of Nicaea’s canons deal with non-Arius related matters, shows that the occasion was taken to attempt to settle a number of other matters with a view to empire-wide uniformity.
Nicaea was not the first church council. The first is recounted in the book of Acts in the Bible. It was called the Jerusalem Council. It was held probably in AD 50, if not earlier. At the Jerusalem Council, it was decided that gentiles (non-Jews) would be allowed to become Christians without having to first convert to Judaism. If Constantine had any sort of model to go off of for the council of Nicaea, this would have been it. The account of the council can be found in Acts chapter 15.