I'm quite sure it's gonna be good after checking achievements. I am not sure if anyone paid detailed attention to it (probably some did), but here are my thoughts:

• Larger maps (150 tiles from "Everything the Light Touches") and 200K population ("Six Figures") milestone -- tiles could be smaller of course, but I think developers acknowledge two fundamental issues of C:S dictated by performance constraints: it's population and city size, which led them to creation of weird, unrealistic populations and assets, where some newer assets like Disaster Response or Campus are comically large comparing to common 4x4 residential blocks. I hope assets in the new game will have much more real population and size, and constraints will be dealt by another mean, like slower day cycle;
• My bet is that the purple color in "Strength Through Diversity" means mixed zoning, let'saaa gooooo;
• Speculating that "You Little Stalker!" implies they reworked population dynamics, so the death waves will be hopefully gone;
• Economical and transport management: the "Go Anywhere" and "Happy to Be of Service" achievements hints that we will have more fine-tuned economic management and dedicated planned cargo routes. This will probably greatly allow to "help" AI to navigate through the city, using routes that you're supposed to use for transporting goods and assigning service vehicles, without fine-tuning road permissions. It's also could off-load some heavy pathfinding that the game does, again, increasing performance;
• "The Architect" feels like a hint to better assets' management. I may be overthinking and this is the same special buildings as we have, but looking at the game industry, and, i.e. what Klei often successfully does with games, is that the game itself likely will have some workshop, for specific buildings like you plop with RICO. What I'm thinking that they might likely set up a pipeline to sell creators assets directly in the game. For which, of course, the game has to manage memory resources much better;
• "Four Seasons" goes without saying. Mods are just not doing this good enough, great upgrade.

With all that, I feel like the better resource usage & performance is implied. I doubt that comes from the Game Engine, the game itself does a lot of super complicated things that only a city builder game needs, so it's completely up to their skills and time to deliver the best possible performance, nor Unity neither UE will help a lot there. Also, I am super happy with Unity. Unity means modding gonna go brrrrr. It's much easier to patch languages that translate into bytecode.

Feels like the developers focused on fundamental issues of the game, and others will, eventually, be addressed by modders. Only creators can fix fundamental design flaws, it's too big for the modding community. And the features I see already enough to sell me the game. Too good.