DA could add whatever creature they wanted and I would probably still be sold. The depth in design, most importantly the lore & history and background of the creatures is indeed fascinating and only game I have seen come close to this is Witcher 3. I love it and try and find every piece of Codex out there. But anyways, the lore is outstanding and my favorite in gaming today. Werewolves, some version or info on Vampires, more corrupted animals or rare combinations of animals possessed by demons or anything of that nature is amazing.
Demons are the scariest creature of all to me on DA BESIDES any of the spiders ESPECIALLY Corrupted Spiders oh ad the huge Monstrous & Queen Spiders which all make my skin crawl. I have such a fear of Spiders in general. But demons make for amazing encounters and epic boss battles in both DA Origins & DA II. In DAI we are introduced to the Fear Demon, which is a hideous creature that pumps out (whaddya know!!!) spiders... I would like to see more info on them too by the way. But anyways that being said, I think if game is set in Tevinter which I have heard it is rumored to be and Bioware has hinted there will be a story to tell one day in Tevinter. But anyways, if a magister is possessed by a powerful demon either Fear or Pride maybe Desire, create a new one more formidable and hideous and scary than we have yet to see. Magisters should provide a demon with a the most powerful body available or presumably. Oh, yeah and add some evil, corr8upt & vile witches, (not mages) but witches who may or may not look Flemeth.