According to riot,
"An Assassin is an agile champion that specializes in killing or disabling high value targets. Focused on infiltration, deception, and mobility, assassins are opportunistic hunters who find favorable moments within a fight before jumping into the fray."
....wouldn't that classify most champions as an assassin than? I'll take 2 strong examples of assassins: Zed and leblanc. Both are high burst, high mobility, and have a way to get in and get out.
But that really isn't the case for all assassins, is it? We have assassins such as Talon and rengar, who while they can easily burst down a squishy, they really can't get out as easily (of course, if they have the ability to burst down a squishy, they usually can kill anyone else in the vicinity)
If thats the case, what defines an assassin? How is Talon any different, from say, renekton? Renekton can go in at an instant, lock down someone, burst them down, and get out unscathed....as a _bruiser_.
What about kled? He has a semi-global presence in an ult, 2 dashes on a single target, heavy burst with a w and q.... once again, as a bruiser.
How about TF? He has semi global presence, heavy burst in a w and q, and if he has E on his next auto, its just more burst. Like talon, he can get in, heavy burst a single squishy target, yet can't really get out as well.
Camille or Vi, or even Yasuo? All are heavy bursting units who have some of the greatest lockdown in the game, who can not only burst squishies, but tanks as well.
Heck, why not vayne or kaisa? They have heavy burst, both have stealth, and since they're adcs, they have to be careful before going in, as they could get bursted down if they misstep.
Fiora, riven, pantheon, rammus, hecarim, quinn, graves, xin zhao, ahri, alistar, I could list off nearly every champion who fits riot's description as an assassin.
If a champion doesn't have mobility, then they have heavy burst: enter MF, illaoi, Darius, Varus, Veigar, Nasus, brand, etc.
If a champion doesn't have mobility or burst? Perhaps they have enough CC to allow other champs to burst em down, like naut, thresh, blitz, nunu and willump, etc.
What if a champion doesn't have mobility, burst, or hard CC? Then they aren't meta. Like Morde, Mundo, shyvanna, xerath (he technically fits into the bursty no mobility, but that's honestly a rarity in my games), rumble, etc.
Now what happens when you have mobility, but thats it? You have Kat.
Point being, I just think that everyone has assassination potential, so what exactly are assassins supposed to accomplish? Why play Diana, go in and suicide bomb 1 for 1,when you could play someone like renekton, go in, kill 3 units, then exit the fight?