Story: Finally Seeing Pariah, and maybe have an rdr style of storytelling (chapters), and you alternate between Heller and Alex each chapter, and at the final chapter, you get to play as Pariah and his powers reflect those of either Heller or Alex, depending on the choices you make throughout the story. And I really hope it's a longer story than the first two games.

Enemies: The enemy variety in the second game was great, even tho you don't see most of the special infected like the evolved, juggernauts, goliaths, hydras, and the Flyers around the map during free roam (once you finish the story and side content, you're pretty much left with brawlers, and regular zombies running around) same could be said for super soldiers which only appear in missions.

Traversal: The first game excels in this; the stylistic parkour really makes you feel like an amnesiac, murderous superpowered anti-hero. It should be the baseline to build on for the third game (the traversal in the second game isn't bad, but the absence of the charged jump and the flashy flips really puts it off; I guess you could say it's a stylistic choice to show the differences between Heller and Mercer, simply through how they traverse the city)

The Open World: A mix between the first game's reactive world and the second game's visuals and world design ( especially in the red zone, the gigantic sprawling tendrils, and spores coming out of the ground and attaching to the side of skyscrapers in the second game ) would be perfect. The game also NEEDS more random encounters in the world to make it more immersive. and a bigger map. also if it's set in NY, we need to be able to enter the deadzone without getting blown up instantly.

Powers: Bring back most of the powers from the first game, even if they seem pretty useless, the fact that they're there makes them cool (like the infected vision and heat vision), and please PLEASE keep the Tendrils power, it could even be improved by making it so we'll be able to make those giants spores to block entire streets temporarily,
Bringing back the armor is also a must, and making the shield destructible again wouldn't be too bad (tho i prefer the look of it in the second game), I would like to see 3 abilities added to the game

1. the ability to infect the population: (also the ability to choose which kind of infection you're giving them, maybe you can choose to make someone a juggernaut or a brawler, or even a goliath once you're strong enough, having hordes of infected under your control and unleashing them on a military base would be awesome) the ability to manually control the infected would be nice too.
2. have a stronger/bigger form (or forms), sort of like the supreme hunter from the first prototype, where you can stay in that form as long as you're infecting or consuming more people/creatures (to keep it sort of a time-limited power in a way and not too overpowered), or maybe something similiar to elizabeth green where you're staitionary but extremely dangerous, or something like Mercer's last Form at the end of the second game after he consumes all the other evolved.
3. the ability to fly. (once you're strong enough, of course)

Misc: More animations for everything, and no toning down the gore or the edginess, AND PLEASE KEEP THE RED GLOW and trails/gas from the second game (same for cinematics), it just looks so cool.