Don't take Assassin's Creed Unity's dearth of modern day segments as a new direction for the series. Lead writer Darby McDevitt says that future games will return in greater depth to the modern day exploits of Abstergo and the Assassin Order.
Ubisoft focused its resources on building Arno's exploits in revolutionary Paris, rather than a complementary modern day scenario, because it was the first new-gen exclusive game in the series. Now that Unity has laid the framework, McDevitt said in a Twitch livestream reported by Eurogamer, future Assassin's Creed games will be able to expand their historical breadth.
Building a new city, or even part of a city, would take six months of work from a big part of the staff, McDevitt said. One of his favorite modern-day interludes, Assassin's Creed: Brotherhood's Monteriggioni, was only possible because the city's assets were already created for Assassin's Creed 2.
Anyway, the plan going forward is to "smartly reuse things so we can have a more robust modern day". You can also expect future games to expand on the history of the First Civilization - you know, those big creepy glowing people who were a big deal in the Desmond games but have quieted down since then.
Modern day, glowing people, and Victorian London, then. Got it.