Space!
I realise this idea may sound like a bit of a stretch but bear with me. Seeing each Assassin’s Creed take on a new setting is a fascinating thing, but there’s something I really like about the idea of Ubisoft taking their endlessly-convoluted Templars-Vs-Assassins mythology and clarifying it through classic science fiction. An installment of the series not overly pressured to bend its narrative around famous historical figures and events could go a long way towards clearing up the confusing nature of the series’ core-conflict and selling players on some sort of narrative endgame for the franchise.
What do assassination missions look like on the Moon? Are there laser-wrist-blades? How could zero-gravity affect the series traversal mechanics? These are all important questions that a futuristic or space-age Assassin’s Creed could answer. Going full sci-fi does take away the historical angle that separates Assassin’s Creed from its competition – but if that’s what it takes to shake up the formula, I say go for it.