I don't think a series having an identity means making the same sort of game over and over. Devs experimenting between games is a good thing, I wouldn't want them to adhere to a strict formula (Lord knows Bioware is already being blasted for that enough).
I do agree they went a bit too far in either direction- DA2 was too limited in scope, and Inquisition was too big and broad. But really, the only real differences between Origins and Inquisition was that the latter had bigger areas, more busywork and its story missions were kinda divorced from the exploration rather than crammed into it, DA2 is the outlier here. If they can take Inquisition, make it smaller, more focused, remove stuff like the shards and lenghten the main quest, I'd be one happy panda. I think the base Inquisition formula works; they just need to refine it.
I mean, you speak of The Witcher. The first game happened mostly in one city, the second had three big areas with a completely revamped gameplay, the third is an open world, and I've seen few people complain. That's more changes than Dragon Age, IMO. Changes in design and, yes, game engine can promote experimentation, and in my books that's good even if it leads to mistakes from time to time.
Yet despite those changes to the Witcher series, the first and second game felt similar to me (granted, I didn't play an extended amount of either game). To use an example I do have in-depth experience with - Mass Effect. Each game introduced some pretty big changes in the series, such as the removal of planet exploration, the inclusion of shooter mechanics such as cover, the streamlining of leveling and equipment, etc.
However, each game "felt" like Mass Effect, due to a shared sense of presentation. If someone walked up behind me while playing a Mass Effect game, they would be able to notice that and have to check to see which game it was. A Dragon Age game does not have that - their art styles, controls, mechanics, encounter design, camera movement... it all feels different from one game to the next. The game's don't share much of any DNA in how they play, other than the story setting.