I feel the same way.
Plan your trip right or suffer the consequences -> which also can be quite fun.
Since my second Fallout NV play-through I always play in 'hardcore' mode. The Stalker games don't even have a different setting.
Both games have some a nerving features, but overall it adds immersion and fun. At least for me.
Of course, both games have a sort of post apocalyptical setting were these features perfectly fit.
In an ME game I'm not so sure, depends really on what type of Character we get.
Do you feel there should be optional hardcore options for eating, drinking, sleeping, hypothermia, radiation, heat exhaustion, disease?
If it fits to the setting, absolutely. But in the current trilogy those features would have been out of place.
I.E. Eating, drinking (and sleeping if it isn't just some sort of 'wait' function) don't fit when you go back to your base/ship whatever directly after every mission (which doesn't even last 3 ingame hours).
But, I'm against diseases in games, at least if they are not directly players fault. IE, took 'drugs'-> addicted. Stood too long near this radiation hotspot-> irradiated.
What about inventory weight and how that effects armour and carried weapons?
I actually like the Me3 inventory system (It's not perfect, but a good start), so - why not?
-> Though I'm not sure if/how to include armour.
Both Fallout and stalker have a progression where better armour gets heavier and uses more of your carry weight, and then suddenly you get an exoskeleton/power armour which has the best protection and allows additional carry weight, (nearly) without drawbacks.
What immersive elements do you like and what would you like to see the game show?
Let me pack my own thermomagazines! -> inventory + inventory weight
Let enemies use their powers again, not just the player character + team (not really an immersive element, i know...)
-I'm not sure if equipment decay would fit with the ME universe.
-Day night/weather cycles are nice if they serve a purpose and the player is in one location long enough to actually witness and use these
-> this is probably not the case when you can visit several planets
What elements do you utterly detest?
All the same, if they are done wrong...
IE the first Stalker had weapons decay and no way to repair them,
FNV has a sleep need, but sleeping places are restricted. (at least I believe so, it has been some time since i last played it).
Generally, every feature/penalty triggered should still leave you the possibility to survive and get back to the base, if you don't screw up any further.
Also I don't like decapitation (at least on the player/ squad). Loosing a leg and sleeping it of is really stupid.