Personally I think the Assassin's Creed series has the most story potential out of any game series available today. You have multiple realities all taking place at once so you can add modern mechanics and old mechanics and work on that. However Ubisoft have been doing a number on the series recently by jumping far ahead or far behind in time on every game they release, it isn't necessary to let 200 years pass in the story to tell the story of another Assassin they can simply expand on what's already available in previous games.

Think about it, currently in the Assassin's Creed series Ubisoft keep adding game mechanics almost as if it's an necessity which isn't true because of what they've already achieved with the mechanics, they can simply work on what they already have for example an upgrade for the smoke grenade could be that the higher the upgrade the darker the smoke plume gets also longer durations and wider ranges. That's just a small mechanic improvement when done right will be a selling point for the game.

Where Ubisoft adds unnecessary things into the game like the weapon upgrades which would be okay if when you upgrade you weapon it gains some sort of physical makeover like engravings or rare metal looks but just for sheer brute power it isn't worth having a weapon upgrade system. Assassins are meant to rely on their physical ability, wit and stealth to navigate around situations at one point in Assassin's Creed Syndicate I maxed out the skills of both assassins and maxed out their gear the remainder of the game felt absolutely boring because I could go guns blazing in every fight and I would always be at a guaranteed win taking the whole idea of stealth out of the equation, it shouldn't be an option but rather a necessity that get's you punished if you choose the weapons hot route.

The system of calling in other assassins to help you is also unnecessary and even more so in the story I will later describe simply because of the fact that they make the game much easier and also takes away from the stealthy assassin way. It is not necessary to add mechanics and weapons in every new game just improve on what you have already.

The Story

So I have some ideas on how Ubisoft can improve the Assassin's Creed series greatly by simply going back to one of the greatest assassin's of all time, Ezio Auditore and build on the story of Ezio but not from his view.
The audience already have a sentiment towards the characters they've met throughout Assassin's Creed 2, Brotherhood and Revelations. You get to see Ezio at birth and Ezio at an old age what better reason for sentiment can you have than that? There's Makaveli, Leonardo Da Vinci and Ezio's family members.

My suggestion is that Ubisoft create a story within Ezio Auditore's story of an assassin initiate who worked as one of his "crew" members when he called assassins in during fight scenes. However with this particular initiate he was destined for greatness through great effort.

The current time story that takes place with Desmond is already in place where instead the story gets told from Subject 16 Clay Kaczmarek's perspective in the events leading up to his demise and the events leading up to him meeting Desmond in the Animus. It could be made so that Clay is a vital member in the Animus program for Abstergo because he simulated the initiate I mentioned earlier as a direct decedent. The initiate could possibly have come in contact with the Apple of Eden before Ezio recovers it or it could be that the initiate was essential in recovering the Apple of Eden by helping Ezio either way works fine.

My vision for Clay Kaczmarek is to be much like Lucifer in Supernatural played by Mark Pellegrino, the wild card type who seems to be near insanity yet is the most open minded in the room. In fact Clay already is much like how Mark Pellegrino plays Lucifer minus the murdering. This character could be expanded upon greatly even to a point he surpasses Desmond in terms of the fan's affection. You have someone who is witty and sarcastic superficially but actually has a strong moral system when it comes to right and wrong which is why he's linked directly to the creed of the assassins.

Some of the main missions are already in place, for one you can do missions where you protect Sophia while Ezio is away because you never learn what happened while he was gone. Another mission could be the initiate being one of the assassins presumed dead when the hideout gets attacked and Yusuf gets killed but the initiate just barely escapes death. After seeing Ezio in the hideout the initiate tracks him down and gets Ezio to mentor him or you can go the route of a revenge plot where the initiate slaughters everyone involved in the act of slaying his/her brothers and sisters but unintentionally made Ezio's job easier by removing obstacles Ezio didn't know he had in the main story of Revelations that would've killed him had the initiate not been there in his blind rage.

There could be a backstory for the initiate that Ezio related to where the initiate lost his/her entire family similar to the way Ezio did. There are endless possibilities to this that is what's so amazing about the time frame Ezio was around. So many characters worth seeing again and seeing another side of the story from a perspective much different to Ezio's.

We can speculate deeply into this, it's just an idea of how open Ezio's world still is and how much it can improved upon especially considering it will all be brought to life with next-gen graphics.