No as i said you'd only be creating a change and then having to add another change to revert the change (for some champions). It doesn't matter if these changes occur because then riot would have to overhaul all of the champions using stealth for very little overall change and that would just be a waste of their time and resources. In the most recent serious overhaul: Ap Items - it did not affect very much in the sense of gameplay it just gave ap champs a nice selection of item builds so that they could build some ap while getting some side benefits instead of being noticeably weaker than heavy ap stacking mages.
Also in your second paragraph you stated that you got a double kill because they weren't paying attention <------- This is not a gameplay error just an error in the player. You can't go around saying imma nerf this and / or give much more counterplay to something if it's just players being oblivious. as i already said stealth is a tool to deceive without this deception stealth is useless: it's what gives shaco his presence it's what allows kha time to reset or juke it's what gives rengar an ability to engage without being cc'd to the ground (and don't pull the: rengar has an indicator - thats due to his ult being powerful- mobility and burst + ferocity build) and finally once again it's twitch's only somewhat reliable tactical tool. You saying people won't pay attention anyway is a very bad reason to do what you ask because if players DO actually pay attention (The LCS for example or hell even just general ranked), guess who gets cut down? champs with invisibility.
In regards to the teemo argument I was using him as an example I am already aware of the buff to his stealth.- No need to bring anti teemo prejudices here. Also teemo's shrooms require stealth and the whole point of his shrooms is that there is usually too many to be completely countered (it is his ult after all). If your changes occur and more stealth defying options exist then this would be made obsolete or require yet another change.
Here's a good point what is the point in having longer stealth durations if it's gonna be completely countered?
Besides haven't you heard / seen at riot developing a singular item for this? The mandrake ward which is in development is a sound ward which creates a noise when someone passes it invisible or not (doesn't give you vision though).
You do realize that anti stealth mechanics (primarily the ability disabling stealth) will screw up anyone who uses stealth mid fight i.e. someone to has the tact to keep their stealth for use during a fight.
By making skillshots / abilities break stealth you have completely removed the viability of this ploy, as stealth is usually used as a way to pull off sneaky plays. Once again if someone is smart enough to get either of the two items i mentioned before in a pre-meditated attempt to counter a champion who they know has stealth they can usually catch them for example for a laner with stealth you just throw the ward down /use the trinket when they stealth and viola you have vision of them and can thus counter them for a jungler for example eve: you place these wards to counteract her not to mention she has a detection radius.
Don't provide ideas that require excessive work for very little change as it's wasted effort that could be applied elsewhere. Ignorant people (people who miss you even when revealed) will always play that way unless they learn otherwise (thus they must make a mistake and learn to overcome it as there are very reliable ways to counter invisibility). I very much dislike play which occurs around other people making a mistake (it's so unreliable and has no skill whatsoever attached to it) because if players play at the top of their form it's far more exciting not just some fool being unaware and then getting nuked for it.
Oh this is one hell of a long post... Sorry bout that.
and
. Any more and invisibility will be rendered obsolete because then no go zones could be established and using invisibility to escape/ gank / blindside an unwary foe as 
for example would mean death as then cc could be used so much more effectively and can be easily predicted (especially for champs which rely on stealth to do their thing).
off completely seeing as he relies on being able to maintain his composure while stealthed (take some hits and still be hidden for an attack). He'd also suffer as he wouldn't be able to stand in the brush next to someone because he has a periodical outline alerting people to his existence (adding true stealth to this means that you've just aimed to make a change to revert another change - it feels kinda pointless).
Spells breaking stealth is a bad idea seeing as it then becomes useless when trying to maneuver to kill someone with lots of aoe especially when ganking a fight.