I'd say that invisibility and chameleon would be a lot less overpowered if enemies could still respond to sounds and other signs of your presence even if you are using invisibilty or 100% chameleon. This would also make more sense than just making you impossible to detect. If you look at the name "invisibility", you should see why, invisibility is the state of being invisible, which means you can't be seen. In other words, if you have invisibility, no one can see you, this doesn't mean no one can detect you through other means. You still make sounds when you're invisible, you just can't be seen. Now, of course, NPCs shouldn't immediately go into combat if they hear you while invisible, rather, they should start to get suspicious, and maybe investigate it, even Oblivion managed to pull off a degree of partial awareness on your enemy's part, it just wouldn't apply once you were invisible. Sometimes while sneaking around, you'd hear NPCs say things like "Whose there?" showing that they thought they heard somthing, but aren't really sure that you're there, they didn't know exactly where you are when they said things like that, but it informed you that you should be careful, or you might get detected. If something like this could be done for using chameleon and invisibility, it could make it a bit less overpowered, without making it useless. Though I think it would also need to be accompanied by AI that responds better to this sort of thing, if NPCs think they hear something, they could go over to investigate, maybe mages could cast detect life, after all, that's probably what players would do if an opponent cast invisibility. Or if they knew you were there but suddenly you vanished due to using invisility or something, instead of just somehow psychically knoing where you are and chasing you to the ends of the earth or just instantly forgetting you were there, they could start throwing attacks in the general direction you were in before for a bit, until they realize it's not doing any good. Aside from the sound of your footsteps, they could also react to things like bumping into objects or walking through water, so when you want to be stealthy, you would want to be careful about that sort of thing.
I'd also say that just like how armor rating is capped in Oblivion (And the cap should remain, assuming Bethesda continues using the current system, as the way it works in Oblivon, your armor reduces damage from all mundane attacks by a certain percentage, for example, an armor rating of 50 means that all attacks are weakened by 50%. Letting your defense go up to 100 would make you completely invulnerable to all physical attacks.) the maximum level of chameleon should be capped at a certain number, high enough to make your chameleon give a significant benefit, but not to the point where NPCs won't know you're there if you're in a brightly lit room, wearing a full set of Daedric armor, standing a meter in front of them and jumping up and down. Alternately, they could still allow chameleon to reach 100, but change the way the calculation works somewhat so that even at 100, your chameleon still doesn't actually make you impossible to see, just as how 100 sneak still doesn't make you impossible to detect. Part of my problem with chameleon is that not only does 100% chameleon make sneak obsolete, it even makes invisibility obsolete. The way invisibility is designed, it goes away as soon as you interact with any object, where as chameleon does not, this makes sense as invisibility has the advantage of making you completely invisible whereas chameleon makes you partially invisible, thus, in heory, both spells have their value. The problem is that once you've managed to get 100% chameleon through enchantments, you can become completely invisible, just like invisiblity, AND stay invisible all the time. I should not need to explain why this is not good game balance. So I'd say we need a way to make both chameleon and invisibilty worth using, instead of making the latter just a weaker version of the former once you can get 100%.
Alternately, if Bethesda doesn't want to put a hard limit on chameleon, they could make it so that once the effect gets over a certain point, the magicka cost or enchantment point cost of increasing the strength of your chameleon by 1 point would increase periodically as the effect got stronger. This way, in theory you can still get 100% chameleon, but in practice, it's difficult, if feasible at all.
Things cannot be overpowered in a single player game. You can make it as difficult on yourself as you want.
No, I'd say that making it so that you can be completely impossible to detect to your enemies, no matter what you do, is overpowered, whether it's a single player game or not. And in any case, if you're going to say the developers shouldn't at least try to make a single player game balanced, then you might as well just put an item in the game that enables god mode as soon as you put it on. As much as I like options, intentionally including the option to completely remove any challenge from the game as an ability available as part of normal gameplay (Using the console doesn't count as that's not normal gameplay.) is usually a bad idea.
Remember, these are illusion effects, not alteration. You do not actually become invisible, you manipulate the senses of those around you so they do not register you.
If they were supposed to effect ALL senses, then why would the spell be called
invisibility, you know, the state of being unable to be seen? Also, in case you didn't notice, illusion also includes the light spell, which, as it's name implies, generates a magical light around you, or your target, or are you going to tell me that it's not actually generating light but just manipulating the senses so that you think there's light? And if your going to say that because it's illusion magic, then it must involve decieving the senses, then by the same logic, shouldn't fortify attribute spells or magical resistences not be restoration either? These spells don't restore you to a healthy state or anything, they enhance your physical or mental attributes, if anything, by that kind of logic, they should be alteration because they're altering some aspect of your mental or physical abilities. Ultimately, the magic schools are all pretty loose, at best. And in any case, games sometimes need to make gameplay mechanics differ from lore and story for the sake of keeping the game fun and balanced. Even if invisiblity, contrary to what its name would suggest, actually makes you not register on ANY sense, it can still be fairly unbalancing. Though the real problem I'd say is chameleon, because invisibility wears off when you do anything, but chameleon remains until the duration of the spell ends, and when the spell effect is at 100 percent, this makes you effectively invincible, as long as you keep sneaking.
If you dont like a magical world, please go play a shooter, but dont ask for elder scrolls to be dumbed down into a linear game where I have no choices.
And when did anyone say anything about not liking a magical world? This is a topic about how to make certain spells balanced better, not suggesting the removal of magic. It doesn't even say anything about removal of choices, it's just a discussion about how to make certain choices, namely. using chameleon or invisibility, less unbalancing. Like I've said, if you're going to use giving the player more options as an excuse to allow the possibility of unbalancing the game to remain, then you might as well just add a suit of armor that activates god mode as soon as you put it on. After all, giving you the choice to utterly remove any trace of challenge from the game would give you more choices.