Revive, Clarity, Clairvoyance - can't we make them useful?
I think Clarity/CV/Revive need to be looked at in terms of their goals and purposes in order to make them worthwhile. Nobody takes them. Ever. But I believe that they COULD be made interesting, with a bit of work. Pretty much everyone can see the issues with them, but... meh. I had a free evening and felt like theorycrafting.
Clarity: When you run out of health, you die. It's a big setback. That's why Heal is a useful summoner spell - at the last second, you can tactically restore an ally's health (or your own) to avert death, allowing for big plays. Running out of mana is nowhere near as significant - it means you can't cast most spells for a couple of seconds. Taking Clarity is only ever going to be a gradual mana-sustain measure, and in that case, why not just take potions or grab Chalice?
If the goal is for Clarity to enhance a team's ability usage by reducing costs, it fails because there are other ways to do this that work better and don't require a summoner slot.
Here's a suggestion for a version of Clarity that allows for big plays, has potentially VERY visible effects, doesn't blindly improve mana sustain, and - most importantly - is worth a summoner spell.
Clarity 2.0: You and the allied champion nearest to your cursor can cast a single non-ultimate spell [U]for free, with no cooldown[/U] (or significantly reduced cooldown), during the next 3 seconds.
Next!
Clairvoyance: The presence of the blue trinket alone makes this summoner spell redundant. Even then, nobody takes the blue trinket. One little spotlight of vision every minute or two isn't worth a summoner slot. It isn't even worth a trinket! (My thoughts on the blue trinket should probably go in another thread, but w/e.)
The idea here, I think, is that you can cast CV to see if an enemy is hiding in a given bush, or if there's anyone coming down one of your gank paths, or if an objective is under attack... but wards can do all of those things. In fact, wards usually do them better.
Every other summoner spell in the game can be used to facilitate big plays. CV can't. Vision is useful, but you can't kill someone with vision. So if you want CV to be useable in big plays, it has to have a much broader use...
Clairvoyance 2.0: Highlights areas of the minimap that contain enemy champions without specifying WHICH champions are there, and without specifically locating said champions. This is more of a "there is an enemy in purple spawn" or "there is an enemy in west jungle" sort of thing - I picture a yellowish haze around a whole section of the minimap. Enemies visible at time of casting, or enemies found up to 5 seconds after, reduce the spell's cooldown.
This one needs a bit of explaining. It doesn't have much moment-to-moment skill requirement, but knowing that you aren't in danger of getting ganked when you want to go in in lane is a big deal. Casting it before a teamfight can tell you whether or not this 5v4 will become a 5v5 any time soon. Entire enemy team is MIA? Better cast this to find out if anyone's near baron. The cooldown reduction mechanism means that it's smarter to cast it when there are fewer MIAs, or when you're just trying to verify an enemy's location; blind-casting is going to cost you in terms of opportunity.
And finally...
Revive: The big reason nobody really takes this is because it has a friggin' 9-minute cooldown. Coming back to life instantly is huge, especially lategame when you have 60-second-plus death timers, so it's certainly worth a summoner slot, but when you have so few chances to use it... Well, most people would probably prefer something they can use at a moment's notice.
The reason it NEEDS a 9-minute cooldown, sadly, is because - like I said - coming back to life is HUGE. Death needs to have an impact. Death is also binary; either you're dead, or you're not. Reducing the cooldown of Revive means you need to reduce the power of the spell, and you can't make yourself less "respawned" with this spell.
Or can you...?
Revive 2.0a: After a 3-second casting time (including a gamewide ping at the location announcing to both teams that you're using Revive), you respawn AT YOUR CORPSE with 50hp and no armor or magic resistance, and with a 50% reduction in all damage output. Any minions, monsters, or towers automatically attack you if you are in threat-range. Over the next x seconds (depending on level... probably 10-30 second curve), damage and resistances will recover completely, and your health will increase to 50% of its maximum, provided you aren't killed again in the meantime.
The fact that you don't spawn at your base and you don't have a speed buff means this is less versatile; this is Revive in the sense of cancelling your death, not in the sense of hastening your respawn. If you cast it, you need to make sure you'll be safe on recovery; death while tower-diving means you won't be reviving, you can't expect to revive to win a lost battle, and god help you if the enemy team has a Karthus. But it means that in the midst of a teamfight, your team could feasibly plan around you being able to come back, assuming they can keep your mostly-dead self intact long enough to do something.
But a miracle's a miracle, am I right?
https://www.youtube.com/watch?v=yokQ0_8__ts Pictured: mostly-dead champion, post revive2.0a.
There's also option B, which is much more tried-and-true:
Revive 2.0b: You come back to life at spawn, yada yada yada. Except that in addition to the spell going on cooldown... based on the amount of time spent dead, the amount of time since the last cast, and the character's level, you have to pay a GOLD COST to come back to life!
Okay, this is literally just buyback from Dota/Dota2. But hey, it works there, right? And while buyback isn't universal in League, I'd say it might be worth giving up a summoner slot to get it.
And if it sucks, well, you don't have to take it. Just like Clarity/CV/Revive are on Live right now.
Alistar walk out of the bathroom, so I know that above quote is full of bovine feces. Vision is useful, yes. It's damn useful. It's the one thing that players don't realize when they face-check a bush or chase a target across the map or asking someone to ward Baron, only to find Baron is gone. Whoever sees the other first has the advantage. That advantage allows you to initiate at your choosing, or to fall back from an engagement even before it begins. Hell, I can't count the number of times where my CV or ward has secured an easy kill for my team just because the enemy thought he was safe and invisible in that one bush. I have also used CV to reveal areas of the map to prevent escapes into the bush, which otherwise would have been a futile chase, or to prevent tunnel-vision teammates from running headlong into a coordinated ambush.

for the amazing amount of extra kill potential you get. About {{summoner:2}} the reason I find it attimes viable is if your playing a chanpion with no skillshots, ex
, because you need vision on them to kill them. Revive is the only one I really agree with needs a buff. I think that the cd should goes down 2x to 3x as fast while dead, allowing for people falling behind to be able to come back. Also, no ability should make an ally spell have no cd or free. Every lb would take clarity for the triple q. Just my thoughts.