There are plenty of interesting ways to go about stealth, depending on what it is you want to accomplish.
IIRC, Lux's W was initially supposed to grant stealth instead of shield. Such a powerful basic spell combined with the magic AoE wombo meta of the time didn't really help the ability pass testing.
Indirect Stealth Variants
Smokescreen like in Counter-Strike
This ability would create passable "fake terrain" like 
Wall of Pain which blocks vision just like a regular wall would.
Click on image just below to see album.
http://imgur.com/a/gAwH1
One-way Mirror
This already exists with 
. His W ability is basically a reverse one-way mirror instead of an actual smokescreen as described further above since you can see inside W while enemies inside W can't see outside.
Mini-map invisibility
This one kind of exists when using 
and 
. The wards disabled by these trinkets don't reveal you on the minimap for the duration.
I've suggested a similar passive ability for an ultimate in some thief champion thread before. It was something along the lines of:
Rank 1 - Invisible on the minimap unless close to enemy champions (~600) or structures (~850). Revealed on minimap for 15 seconds when pinged by an enemy.
Rank 2 - Invisible on the minimap unless very close to enemy champions (~300) or close to structures (~700). Revealed on minimap for 12.5 seconds when pinged by an enemy.
Rank 3 - Invisible on the minimap unless very close to structures (~550). Revealed on minimap for 10 seconds when pinged by an enemy.
Having vision of someone on the minimap is actually a very powerful thing and such an ability can exert immense amounts of pressure even on very coordinated, skilled opponents.
Direct Stealth Variants
Peripheral Vision Stealth
This is a modified version of 
perma-stealth.
The champion with this passive or active ability would be considered "in stealth" if no enemy champion is facing them.
Of course you can tweak the values pretty easily. As long as you are moving within about a 140 degree arc of 
, you gain the passive MS boost. This stealth would work in a similar way. You could have it scale so that initially, you can see this enemy as long as they are within a 140 degree arc of your champion or allies' vision. As the ability ranks up, the arc gets smaller, maybe to 120, 100 and down to an 80 degree arc at max for a passive.
In extreme cases where this is a short-duration active ability, you might even reduce the vision arc detection range down to something like 45 degrees which is barely larger than cone-shaped abilities in-game. Once again, this can be quite powerful even against very coordinated and skilled opponents given that champions get lots of click inputs that change their direction of facing constantly.
Predator-style Stealth
If you're familiar with the Predator movies or stealth in Heroes of the Storm, you'll know what I'm talking about.
http://www.mobygames.com/images/shots/l/386726-aliens-vs-predator-requiem-psp-screenshot-stealth-mode-in.png
You will be able to see the stealthed enemy if you're paying attention or looking for the slight distortions in vision.
http://allthisrunningaround.co.uk/wp-content/uploads/2013/11/predator_camouflage.jpg
This type of stealth works very well in a game where skill shots are prevalent since an observant player has options for counterplay.
Stealthing Enemies
Technically, 
can do this to an extent. He is able to make enemies untargetable through his ultimate, which when you think about it is one of the things that stealth provides isn't it? It is also the primary reason you would want to hide an opponent from their allies, so that they can't be helped simply because they can't be seen.
Actually granting an enemy stealth is very tricky business balance-wise because there are ways for it to be abusive and for the ability to just not feel good.
What if you do grant an enemy stealth? What are the implications and interactions? Are they stealthed to their allies and you or revealed to you? Does them attacking or using abilities break the stealth or is it a time-based thing? What about when you attack them? Are their AA, spells, summoners and or items disabled or usable for the duration? Does an allied AoE or skillshot which hits them while stealthed reveal them? Do their allies' auras still affect them while stealthed?
As I said at the start, it all depends on what it is you want to accomplish. If you have a very clear reason to stealth an opponent or something thematic, then of course a way can be found to accomplish it. But for as long as it is vague, it stays complicated.
