Grenades as they are in TF2. (+Ideas.)

Part 1. Classification.

ALERT!!! OBVIOUS THINGS WERE WRITTEN BELOW!!!

You know, that there are no TFC-style grenades in TF2. But there are another grenades, such as (1-9th classes):
1) Mad Milk; Sandman's Ball;
2) -
3) -
4) Grenade Launchers' Bombs;
5) Sandvich; Buffalo Steak Sandvich;
6) -
7) -
8) Jarate;
9) Fake Corpse.
Also, there was throwable Sapper in Meet the Spy, that exploded minds of thousands.

Mad Milk and Jarate works the same way: debuffing enemy by buffing your shots, which will give you HP or mini-crits. I want to tell obvious thing: it's the same thing, because the only important thing in combat is difference between your health and the enemy's one. So, you can add HP to your heatlh bar OR you can deal additional -HP to the enemy's health bar. I haven't looked to the numbers of how they works exactly, though it's only about the idea. I wish it's balanced with other speeds (for different classes), that's how it should work.
(Thoughts about Soldier's Banners will be told later.)

Sandman's Ball works a bit another way. First of all, it can be used a lot of times, if you pick it up after use.
Secondly, it depends on the range* it hits enemy (the further, the more different effect it makes). Also, it deals low damage. Effect is: debuffing your enemy so he can't shoot to you, but you can deal damage to him. This effect "takes" enemy's life-time and gives it to you. That's how it works in terms "Difference in HPs".(*I won't speak about what I think about the Sandman now and 2 years ago - it'll be special post for it, I guess.)

"Bombs". Now we have 2 different kinds of the Grenades: first grenades explodes by touching enemy and after the timer is over; second grenades explodes only by touching enemy (no grenade jumping). Effect of these grenades are the same: damage. The biggest damage you can deal by non-critical not-going-to-head projectile.

Sandviches. This works very simply: you take additional HP, so the "Difference in HPs" works here like in Mad Milk's case, but here you don't deal damage to enemy. It's like Grenade, working only for one side.

Fake Corpse. Players were waiting this one a long time and also there was modification to throw your fake corpse (before this feature was released officialy as a Dead Ringer). There are 2 different Fake Corpses - BLU and RED...I mean - the enemy's mate corpse and your mate's corpse. You know, it works 2 ways:
1)After enemy has shot to you and seen your, Spy's corpse, he will ask himself very fast:"Where did this Spy come from? Have I killed him before? Hmmm, what kind of Cloak does he use?", etc. You have time to run away and return later to exterminate this victim anyway.
This kind of "grenade"(both RED and BLU fake corpses) in term "Difference in HPs": it works discrete(not continual) way, i.e. OR you have HP and enemy has no HP, OR you don't have HP and enemy has HP (alive-dead). Because you use Dead Ringer not to be face-to-face with your enemy (I can't imagine Spy that uses DR with any revolver, shooting to the enemy and using DR in rotation, except rare situations when he wants to change angle of his shooting while being invisible ).
2)If enemy sees your mate's (fake) corpse, he will run for the next kill away. In most %% of situations, if you've properly used DR this way. Obviously, he WILL shoot to you. So, your HP will lower. Then, again obviously, you will use DR anyway, not undisguising  and not starting to shoot (because it's weird to use this kind of disguise and then begin shooting/knifing), only if enemy has high HPs and there are other enemies...Ok, about corpse...it is not your corpse - it's the corpse of the mate you were disguised like.
It gives less paranoia to the enemy, but more fake information of your team classes (again - if you did all properly). And this is the only difference between RED and BLU fake corpses. The similar part has been read above by you.

[IF you have any questions/criticism about Part 1, please, ask/discuss in comments.]

Part 2. Ideas.

And now is the part of ideas. Pure (not Poor) Theory.

NB: Grenades can include solid, liquid and/or gaseous materials. 
I start with the previous article of the Part 1 - "Fake Corpse".
As you know, there is only two kind of corpses: your team colour's corpses.
So, there could be added new option for the Dead Ringer (or just sidegrade of the DR?): the corpses of your enemy's team colour. How does it work? Well, you just imitate death of the enemy's mate (any kind of the death, that depends on the shown HPs of the disguise). It helps if your mates imitate killing you. "Oh, my friend was headshotted! This is dangerous place!" - enemy will say... Imagine other sides of this weapon by yourself ;) Just the part for using it: activate DR and press M1(LMB) to "shoot" fake enemy's corpse. Be aware not to do it right before enemy starts shoot you (you can't do it after he starts, you know).
Now about other classes and their throwable "grenades".
2), i.e. Soldier. (Rockets are not grenades that fly like a sniper bullet, but slower; in this way we could end with discussion that Soldier is Demoman-unlock, so don't start it!) Now I just can put this idea: grenades (visually like on his chest-belt or in the Equaliser' taunt) with choosing the delay of the explosion (yes, like in Worms).
As I see it in-game: ver.A) Choose grenade and, holding M1, press 1-5 to choose the delay that will be shown on the grenade (or on/in another surface/place), then release M1;
ver.B) Press M1, then you will see seconds you have before the grenade explodes. Also, Soldier can deny the activation of the grenade, inserting the grenade's cotter back to the grenade. Replaces 2nd slot, like most throwable replacements.
3), i.e. Pyro. First idea that comes to every brain is "Napalm Grenade[that sets enemy on fire]!!!1". Kill this idea - you have it in TF2, just use Flaregun to understand it. Any Grenade must be thrown to the ground?! Such a rudiment of thought!
Second idea that comes to less brains is "Make afterburn longer!" Not so bad idea, but it works only for Pyro. It would be better, if it makes DoT(Damage-over-Time)-effects, such as Afterburn and Bleeding, longer. Ok, last step here makes it even better: it could make oT(over-Time)-effects longer. You know all of the oT-effects: Afterburn, Bleeding, Jarate, MadMilk, Sandman's Ball "Stun", Mark of Death. That's the idea. The visual part isn't any flammable liquid in the bottle, is it? Also, no numbers in the way "it makes oT-effects x2/x3/x1.5/x4/xN longer". Else, I think that if it makes negative(or:positive) oT-effects of your mates shorter (or:longer), it is very useful. Too very useful, so it comes overpowered and it doesn't fit Pyro as (mostly) offensive class. This option would be better for Medic, support class ( V I'm speaking about it below V ).
(Third idea is "vsClass"-weapon, that let Pyro burn another Pyro if last covered by this liquid).
6), i.e. Engineer. I just have started working under the concept and visual part of this one, so there will be big post later. Next class!...("Wait, how about throwable Mini-sentires?" Ah, nevermind, it's common dream!)
7), i.e. Medic. No "Holy Grenades of Healing"! No poisoning(new DoT-effect?) grenades!
Just the idea marked below: "make the teammate's effects-over-time(without damage) longer and the enemy's effects-over-time(that are affecting your teammate) shorter". If it replaces Medigun, it's balanced, as I think (another Support role, mixed with Offensive). Visually, this grenade could be jar with label "Placebo" and there will be nothing on the Medic's back.

Part {1+1/2}. Soldier's banners. Possible Ideas.

 These are not grenades.

The banner works on area, touching teammates by it's effect. It grants or mini-crits(Jarate-style), or additional resistance(no similar grenade?), either healing(MadMilk-style). Also, it works against multiple enemies and it charges like an Uber (by taking and/or giving damage).
First of all, all 3 charging-types has been made in these banners. It could be done new one only for new kind of effects (that are not "giving more/taking less damage//healing" too). That's why I think we will not see any new Banner this year (just my opinion, which is covered by my thoughts). Situational banners? Charging by rocket-jumping?. Not the time to think about it.
Secondly: you have grenades after you respawn or after it has been "reloaded"**. The banner's effect isn't available at the beginning. So, it's the possible way to create new kind of banners - with 100% at the start. Faster reloading? Faster switching weapons? That could work, but though it's too powerful even with small values of the effect's stats.