Thank you for your response, Aenra! My main goal is indeed not any major overhaul to the spells in the game, though some alterations and balances are necessary to both fit the story, and some quality of life changes such as passive Spirit Vision. My main goal is to add replayability with new stories, characters and places to explore in the main campaign.
Which also hopefully answers your question, Endarire! It is for the standard campaign, so all the origin characters are available, but it's best to start a new save when the time comes
Returned to the north for some further work. Pretty good progress in town.