I'm making a sims-clone, as I've said numerous times already. I'm not slavishly re-creating any one of the games (though I often feel like I have to at times for practical reasons, on a side note designing this thing I've actually gained a deeper understanding as to why The Sims 2 worked the way it did).
Possibly one of the biggest problems these games have is that things get boring once you have everything. When you start off, you're playing fully adult characters who don't have a single damned skill or anything other than enough money to build a starter house. Getting to the top is quite an effort. However, once you're there you're so swimming in cash the whole thing becomes a joke. You also have all the best items in the game now too, and any other characters born during gameplay have almost the equivalent of an adult's life span before they become an adult themselves. This combined with all the best furniture items being already present for them means they can easily build themselves up far faster than the founding generation can.
I've thought of several ways to fix this, but I'm not sure if they would be sufficient. Here are my ideas:
Once you're wealthy, there's nothing that can bring you down. Living expenses are trivial even at the beginning of the game. Even if you made no money at all, after the first generation dies you can easily survive without any other character making a single penny their entire lives for several generations before the funds the founders built up runs out. The solution to this would be to increase the cost of living. If you have a mansion, you're going to need multiple characters at the highest tier of their career paths to pay the taxes on it. If someone isn't pulling their weight, everyone's going down. The repoman is going to be taking your stuff, you're going to struggle to buy food, your characters are going to be unhappy about not being able to buy the stuff they want. So higher taxes, and maybe higher food costs. If you have the lowest wage possible, you should only be able to pay the taxes on a minimalistic shack that only has the bare necessities. No fancy house, no expensive items. Not even enough to support a family. You want to move up in the world, you gotta be making more.
As mentioned, another problem is the items. Items that help build skills come in over a dozen tiers, each one being progressively better at building a specific skill. Once these are all present, building skills becomes trivial. Even then, once you have all the expensive items, your sims are never going to roll wants to buy more. The solution to this would be to make items have a finite lifespan. Those dinner chairs the founder bought aren't going to still be usable after 10 whole generations. At best, items should give out by the time the grandchildren come of age, meaning they at least will have to build back up themselves. More expensive items should last longer of course, but nothing lasts forever. This includes items such as career rewards which are far harder to get; unless you have multiple characters in the same career track, you're never going to get multiples of them.
Increase the time to takes to build skills. As-is, you can easily max all the skills before you grow old even if you start as a full-grown adult. Something like that should take one's entire lifetime. Also, such a character would probably have to stay out of work so they would have enough time to build up all these skills. For practicality though, a fully adult sim should still be able to reach the top of their career before their time comes, but of course that normally doesn't require one to max all the skills and get every 'power' in the game. To fix adult characters, some sort of system should be implement to make them have skills and friendships as if they actually did have a life prior to the game's start. Then the game can easily be balanced without making it impossible for pre-made adults to achieve their dreams.
Discourage wealth-building. Even if you do want to just lay back and live off your inheritance, its hard to do that. Sims have a feature called 'lifetime wants', which is simply an ultimate goal they want to achieve. Problem is, its rare for this to not be 'reach the top of x career'. Fortune sims in particular just want to make money by any means possible. The particular curiosity is knowledge sims. They're supposed to want to learn things, but outside of 'max all skills', all their possible lifetime wants are just 'reach top of x career'. In fact, they have far more of these than even fortune sims do! If you aren't making money, its hard to keep your sims happy. Wealth doesn't buy happiness; simply having a giant house full of expensive stuff isn't actually going to do much for your sims' moods unless they really like to build skills. Even then, all skills can only be maxed once, so once that's done it can be really hard to keep a knowledge sim happy. They just fall into a cycle of wanting to witness the paranormal, which is not a common occurrence. Its okay for fortune sims to want to become a boss all the time, but it shouldn't be the norm for everyone else. Popularity sims should want jobs with short hours so they can spend more time socializing. Knowledge sims should want this so they can spend more time studying. Family sims should want to stay home and interact with their family. Romance sims should want the same thing popularity sims do in terms of jobs. Pleasure sims should also prefer to party than work. Everyone other than the fortune sims shouldn't be a workaholic. This however would require the addition of far more stuff to accomplish. Like I said, knowledge sims' moods tend to tank once they've learned everything simply because they've done everything they could possibly want. There should be more to learn, and more interesting things to do with all the skills they got. Maybe I could also make it so characters aren't going to be rolling lifetime wants that make money if they have a lot. If they have a lot, they're going to want to relax and waste time with all the expensive stuff they have now. They got money, they shouldn't be interested in working. Only fortune sims should want work all the time, and only they should be rolling wants for promotions one after the other. Just because they could still advance they shouldn't normally focus around getting the skills they need to advance another level even if they clearly have no use for the money. If they want money, it should be in an indirect way such as wanting a better telescope or something. If you don't need more money to do it, you shouldn't be seeking more wealth basically.
Less free money. The game is actually quite generous with donations. If you have one of the later expansions installed, an npc will come to your house within the first few hours of gameplay and give you a highly expensive computer. Many challenges actually forbid you from selling this item. Getting promoted also gives you a ludicrous bonus; often several times that of your salary. Its actually the main reason why sims can build wealth so fast. Then there's dating. If your sim has had a successful date with someone, there's a chance they're going to come to your house and gift you an ultra expensive item you can easily sell to get a boost to funds equivalent to a promotion. Clearly, all such things must be removed; if you get a bonus, it should be for doing something clever at work. If you want to save up money, you're just going to have to rely on not spending your salary. If you want to accumulate wealth, you're really going to have to work hard for it; its not going to be handed to you.
Of course, at the end of the day this all may only delay the inevitable. No matter how hard you make it to accumulate funds, sooner or later you should be able to become trillioinaires. If you're making even slightly more money than your taxes are eating up, its still going to gradually accumulate. Items breaking down would probably help the most, since it will always be setting you back to square one. Honestly though, such a thing could be annoying, especially since you're likely to have purchased a bunch of stuff at the same time, meaning that one day seemingly everything in your house is going to fail, and if you have a lot of items and/or they're just expensive you probably won't be able to replace everything, possibly causing major problems for the family out of nowhere. Skills are another issue; how do you determine what skills a pre-made adult should have? In the sims 2, you can easily max all skills before adulthood if you try. Why then is not everyone fully skilled? What would be the typical level of skill for an average adult character? Should I place a cap on how high skills can go before the adult stage (in the sims 2, toddlers can actually max out 3 skills pretty easily, seriously). That seems kinda heavy-handed though to just say 'you can't progress past this point until you're an adult'. Most skills are tied to personality traits; the personality values affect how fast you can level skills. Maybe I could also use them to set a cap? If you're not an adult yet and you only have an outgoing score of say 5, you shouldn't be able to raise your charisma past that until you've come of age. There just doesn't seem to be a good solution though.
The things causing this one problem are numerous. All games seem to suffer from this; once you've got everything in Skyrim or Minecraft, the game becomes boring. You can't possibly die, and you have everything you could ever need to do whatever you want. There's no way for you to fall back down once you're at the top. Maybe this problem is just impossible to solve?