Well I dont know the first thing about coding (I just paint stuff), so didnt want to get carried away, but yes I did wonder about how they might accrue temporary stat maluses in the tactical layer. Assuming the code allows that to even be implemented, you could take it in any number of directions. Less energy seems obvious, less reaction maybe, and everything besides.
As for implementation, I'd presume it could be done like in Longwar where they get flagged by the game as injured and are therefor benched for a number of days derived from their stats. I would presume that if the game can derive the number of injured days from damage taken, it could just as easily be told to check fatigue against other stats (morale, energy, mission score, mission type). If it could be scaled against stats, I wonder if it would have to scale linearly or if you could put a curve of it?
Or you could sideline the stats entirely and enforce a hard limit from the moment they land back at base. I'm not too keen on tying it to turns, but the idea of a disastrous drop burning out your troops is interesting. But then again I suppose a disasterous drop is going to leave them with injuries and morale hits anyway, so maybe that's enough. I dunno, I'm just spitballing.
To me it seems the hardest part would be balancing....given how quickly missions can appear in the late game in old x-com as opposed to 2012 xcom, the recovery time would need to be much shorter. I'd strongly suggest a day or two, although if multiple-drops per flight are ever possible I'd suggest it scales with that. 2 missions back-to-back equals double the down time, etc.
Also, this means any given base would need an A-squad for big missions, a rapid-response team kept on backup for unpredictable situations, and probably a full squad of security guys in case of base assault. Lots of guys. Dunno if that'd necessitate a cost rebalancing, but I always ran with 100+ troops in vanilla and still made more money than I could spend. Always found that there was still plenty of experience to spread around the men too.
Worth noting that it necessitates good balanced recruitment too. It's no good just having lots of bodies, you want some redundancy built into your roster. Have spare specialists. And while you want them to perform their designated role well, you also need them to be able to fill in for benched men. Personally I always have a pretty standard deployment (two 5-man fireteams with good aim, mobility and reaction, and a base-of fire element with the rocket, a loader, and autocannons...all strongmen, basically), but this'd force me to either adapt that setup or make sure I have enough appropriately skilled men to fill it.
Mainly I was just wondering if anyone more familiar with the code could confirm whether it is theoretically possible. As I said, curiosity.