Here's my take on this, Bosses are optional or required. I don't see any need for a middle ground. To be able to fight Aldia, one must defeat another optional boss (Vendrick) which could be argued that one must defeat the Ancient Dragon as well for the 5th Giant Soul. Now, I know Ancient Dragon could be avoided, but the usaul route through to killing Vendrick usually consists of reducing his defenses all the way.
So, if Aldia were a semi-optional boss, would that make Vendrick (and possibly Ancient Dragon) semi-optional bosses as well? I don't feel that Aldia is anything more or less than an optional boss, I think of it as a more "true" ending when we do kill him, but I don't believe he causes enough impact to be anything more than optional or bonus.
However, with the Shrine of Winter's flexible opening criterion, I do feel that the Great Soul bosses could be considered "optional." I'm not proposing that we start labelling any boss that limits our ability to reach the end optional, but I do just want to throw that out there. With the Shrine of Winter's requirements, the only mandatory bosses would be Last Giant, Dragonrider Duo, Looking Glass Knight, Demon of Song, Velstadt, Guardian Dragon, Giant Lord, Throne Watcher and Defender, and Nashandra. However, it's generally accepted that the "true" way to reach the end of the game is via the four Great Souls.
Okay, I've rambled enough.About Aldia and Priscilla, I think we should just continue to label them as optional bosses. The credits can be reached without ever fighting Priscilla (or even stepping foot into the Painted World, for that matter) just as the credits can be reached without the player ever fighting Aldia and going through his list of requirements.
I'm sorry for the rambling as well as the essay, hopefully this will actually contribute to the conversation and not just confuse and/or disturb thought.