There are a number of players who wish for more realism in the sims, but how real is too real? How do you add realism to the game? One of the biggest changes was from Seasons, where a weather engine was implemented. Other expansions mostly added a variety of activities and interactions for sims, or in other words, stuff to do.
Along with Seasons, Illness made it's return to the franchise (unless you count nausea as an illness, but that's been going on since the base game as a result of either morning sickness or eating spoiled food). Thus far I've only had my sims encounter colds, although getting the flu is supposed to be possible (it just takes alot of neglect and I haven't been able to do it before the 'germy' moodlet expires). It would've been an added bonus to have sims suffer from headaches (because drops in atmospheric pressure can do that, and it's directly linked to the weather). Injuries were added instead to the Medieval spinoff series.
But what is it that makes injuries so difficult to implement?
Well, there's the countless animations across multiple eps (not all of which the player may have installed at the time) that would need to be redone, or would they? If you consider how casts today are different from years ago (waterproof, stronger, lighter, customizable), it may be possible to work around some of the limitations.
I fractured my arm once when I was little, and ended up wearing a sling for a couple of weeks. If it hadn't been my left hand I'd have been alot more limited, because a sling is designed to keep your injured limb as close to your body as possible in order to protect it. Years later, one of my friends broke her arm on the playground. She was fitted for a cast instead, and was still able to move her arm around because the cast only covered from her hand/wrist to just below her elbow.
When you consider casts like that, you wouldn't necessarily need to remake animations. The only problem you might have are animations where the wrist moves, although you could also grey out certain options to keep the sim from performing them. Kind of like how pregnant sims can't do certain things (for obvious reasons). The actual cast could be wearable in the form of an accessory.
In the case of a broken leg, a walking cast may be a substitute for a traditional cast, since they allow you to put more weight on that leg. If you choose a design like this, where the cast covers the area below the knee, you may not need to change the walking animation. Obviously though, running, jogging, and otherwise bouncing around are going to be out.
Alternatively (or possibly additionally) crutches and/or wheelchairs could be an option, similar to how canes worked in Generations. You just choose when to start and stop using them. Not the fastest or most efficient way to get around, but it makes for an interesting prop for storytelling. Plus it's just fun to harass people with canes. These however would require additional animations.
The next big factor is how long a sim would take to heal. Some injuries (like my fractured arm) can heal in a week or two. Others take a bit longer. Factor in how long the player makes sim lifespans, let's say 'they've got 'short', and you obviously aren't going to want an incapacitated sim for very long, particularly when they live alone. Albeit, the new roommate feature coming with University could help this issue if the player is willing to put up with NPCs in their household.
If I had to hazard a guess though, I would say that an injury shouldn't take longer than a day or two to recover from. Remember, it takes a sim mother 3 days to go through all 3 trimesters (male sims who've been abducted only suffer for 2), so 3 days could be like 9 months.