You're probably not looking for an answer like "it has to be in first-person," right?
Well then, in my opinion, the must-have features in an FPS are:
- A way to tell how much health you have
- A way to know what weapon you're currently using
- A way to check your remaining ammo
The key phrase here is "a way to know." Sure, it's easy to just slap everything on a HUD, but honestly, I don't think it has to be a HUD—as long as the information is conveyed clearly in some form.
For example, in the classic DOOM, there's a face at the bottom of the screen. Even without numerical health indicators, if there are enough expressions tied to different health levels, you can get a rough idea of how bad things are just by looking at the character's face.
So as long as players can roughly tell their health, weapon, and ammo count, a HUD might not be necessary at all.
If those elements are well-designed, I think that’s enough to make a first-person shooter work.
And if I had to add one more thing, it would be a sense of playfulness.
Since you're working on a sci-fi game, I think it's essential to include little Easter eggs or references that only true sci-fi fans would catch.
For example, maybe the tutorial AI is called "SPRING 9000" (a twist on HAL 9000, with "HAL" being read as "Haru" in Japanese, which means "spring.")
Or maybe the aliens greet each other with a pose that’s unmistakably the "live long and prosper" sign.
Maybe sharks rain down from space.
Or in a world full of futuristic tech, the clock is weirdly analog.
Details like that absolutely belong in a sci-fi FPS, in my opinion.
That's about it from me!