I want to start off by saying that Im a huge fan of the assassins creed series, or more accurately was, as I have played every game in the series leading up to origins, before I had not played the franchise's new releases(orgins, odyssey). And on top of that I'm incredibly excited for AC Valhalla, I'm a big history geek for the viking age and this setting, so even with a bit of myth mixed in, like most ac games, this is the first AC game I will be purchasing in some time.
Now onto the part regarding my opinion on where the series stands today and why I wouldn't buy the next one if it were not a setting I was so thouroughly interested in. Having played every AC game in the series up until origins and odyssey, I've come to love and dislike many of the things about the series, one of the things I loved,  and I think most people did not like this part, were the modern sequences of the story in the earlier games, and the fact that this modern timeline with desmond and others, was the focus of the main, alternate historical/sci-fi aspects of the game and its roots. That being said, if you have played the games up to AC:3 with Connor, and the last game including Desmond, you would know that AC:Black Flag took the story in an entirely new direction and made up some lame scenario where Juno, of the first civilization, could not do the things she was going to do at the end of AC:3.
Dont get me wrong, Black Flag is one of the best games in the series in terms of gameplay,  and the Idea of being a worker at abstergo with the new animus tech could have set up some great stories for the ancestor you play as, and the person you are. But this was the game, as a longtime fan of the series, that I became truly upset with the creators of the franchise for abandoning such an amazing and original story that was the basis of all the games that did so well before it. To me, this was the game where they said "scrap it" to the idea of continuing some sort of story that would come after AC:3, and this was so disappointing to me. And from that point on, some of the games were great in many many aspects, but now... we found ourselves spending 95-100% of the game inside the animus as our ancestor, and almost never experencing anything outside of the animus, or learning anything relevant about the modern world and the goal of experiencing this synchronization.
Now, I'm not saying I would like to see a return of the parkour and puzzle actual gameplay in the modern world, but I have never been able to get over the fact that the story was left behind with such disregard and carelessness to what had come before. As I said, I havent played origins or, odyssey, so perhaps I could be wrong, and since syndicate and rogue they have returned to elements of this part of the story, but from what I have seen this is not the case.
Of course there is the regard for historical accuracy too, and I myself dont have a problem that they included mythical elements and monsters in the more recent titles I havent played. To me, these elements were the basis of the franchise, these alternate biblical history and sci/fi elemnts in a beautiful mix of story. And nothing has compared to being Ezio, holding the apple of eden, and finding it later as part of the creed in the modern world.
These are the moments and parts of the story that I truly miss when I played Black flag, and Unity, and Syndicate. We dont get to see the repurcussions or the goals of our time in the animus anymore, and I feel like there is SO much lost potential in the series and the story that could have come after the events at the end of AC:3 and the continuous battle of the templars and Assassins through history. Since this moment in the story has taken place, now what we have is yet another oppurtunity for ubisoft to make a franchise that allows them to make the same game every single year, with a new setting that will sell, in the same vein of games like Far Cry.... and if its vikings this time around.... I'll buy it begrudgingly.
TL:DR; I am a longtime fan of the Assassins Creed series and I wish that the narrative directors would return us to the modern story between the tempalrs and assassins, that was the root of the original games and their story.