That is just matter of interpretation. A blood mage will fit in anywhere. You just need people with writing skill to do it. This current story line is just too rigid for it. Holy person whom has to feel holy. Where is the rp there? Had the DA team had some balls they would have made a game with parallel story lines, including the one with blood magic. Even you should be able to see how many possibilities there are for story variations.
Parallel story lines and story variations are precisely the things game developers want to avoid. Creating them takes a lot of effort, while the impact on sales and developer reputation is much smaller. The ideal game gives the player an illusion that their choices matter, while having a minimal amount of alternate content.
The biggest issue with having blood magic in DA:I is continuity. In order to maintain continuity between DA:I and the hypothetical DA4, DA:I must always end in the same way, regardless of player choices. There can be trivial variation, such as what happens to the Grey Wardens, Templars, and Mages, or who rules Orlais and the Chantry, but the core world state must always be the same. Corypheus must be defeated, the South must be more or less in peace, the Inquisition must become a major power, and the Inquisitor must survive and continue to lead the Inquisition.
Because blood magic has been established as the ultimate evil in Thedas, this rules out the Inquisitor as a blood mage. If we have a powerful Inquisition, its leader cannot be a hero in some world states and a villain in others. A DA:O-style ending, where the main character's story is essentially over, would have allowed more freedom.
So you think. Unless you are part of the DA group that is just your opinion. Spoken dialogue limits only as much as the skill of of the writers. The cost of it is hardly a issue if you use voice actors who are willing to take less than known names. Doesn´t mean they have no talent for it. And frankly I will take more richer story with rp option over those empty grinding maps of DAI any day. Even it would cost more in the price of the game. This is the first DA game I had problems finishing in my first playthrough.
The issue isn't with writer skills or actor salaries, but with the sheer amount of work. According to the published figures, DA:I has about 82000 lines of dialogue. That's 20% more than DA:O, 30% less than BG2, or the equivalent of 10 seasons of Game of Thrones. Recording it in just one language might take around 8000 hours of studio time and a similar amount of postprocessing. If you want to change the details of a scene, you'll have to get the actors back to the studio to record the new lines.
Ultimately the problem is that we're far less forgiving with speech than with other assets. If we have a piece of text, a 3D model, a map, or a sound effect, we can easily generate minor variants of the asset. With speech, every variant becomes a new line that must be recorded separately. Eventually speech synthesis will replace most voice actors and solve the problem, but we're not there yet.