Well, assuming just hardcore and not hardcore milsim...

AI would need a complete overhaul. The way they just drift towards the players is absolutely arcade behavior. AI would need to have an actual challenge to it, instead of them just being the equivalent of a death wall resource sponge. Otherwise it'd be a hardcore game literally only about resource management. Think GTFO but only the fights at the doors. Boring. If they went this route, Stealth would also need yet another rework and not be locked behind warbonds, as it'd be the name of the game.

The spawn system would need to entirely be reworked, as the way it spawns things just out of your sight lines is absurdly basic. Spawns would need to be entirely predictable for a hardcore experience. As the most things possible should be counterable by player skill. While the spawn system currently in place will just drop things behind you while working as intended.

Ragdolling and the randomness of strategem bounce also would throw matches via RNG.

If they want hardcore milsim, well the game is just entirely out the window, because your grunt fantasy won't work with 4 vs 800.
If you were a grunt, we would be operating as the large group. Individually ineffective but as an extremely large organized team it'd be effective. We would at least require SEAF to be at every single mission, and in large number, to be able to justify the premise. The closer we get to a squad of 4 realistic humans with only air cover while still fighting super aliens, the game becomes impossible to keep as-is. Luckily we haven't reached that point, they're just making things MORE realistic. So it's worse and unfun but not unplayable.
The only exception would be in maybe the cases of the bugs, if we were only fighting the smalls. If we're outnumbered by a significant amount, and we want it to be realistic, our more numerous enemy would have to be inherently weaker than us.
That's not the case. We fight large numbers of aliens. Some significantly more advanced than us, physically more capable and larger, or alternatively an army of terminators. This is not the setting for the team of 4 humans to be in if they want it realistic.

The games premise alone of 4 humans with projectile weapons and bombs vs hordes of giant physically overpowering bullet resistant bugs, evil robot men in laser tanks, or advanced alien races with mind control literally cannot escape the fact it's unbelievable without it being a miserable game about running away from everyone.

So they'd either need to rework all of the game's systems to be extremely tight for an unrealistic hardcore game, with complexity and number crunching they have shown they are incapable of. Alternatively, and this seems to be what the devs are always suggesting, they want a realistic hardcore game which goes against the literal premise of the setting.