Quote from Jhfffan>>

For the bright blade (and any others that deal extra damage as part of an attack), you may want to consider the language as the creature takes extra damage as opposed to the attack deals extra damage. The reasoning is that "the attack.." is eligible for crit upgrades where "the creature takes" is not. Neither GFB nor BB are eligible for crit damage because of the way that they are set up and these should be similar unless they are balanced around the idea.

I want Bright Blade's secondary damage to be able to double on a crit, that was a design choice. That's part of the reason why I reduced the damage dice. I don't think it's OP to do so, especially due to crits being relatively uncommon. 

The Shock Blade could be centered around crits by making an attack by another creature against the target crit on a 19-20 while allowing the attack to deal an extra d4 damage per step.

I'm not sure what you mean here. Can you please clarify?

The withering blade would also be a candidate for the 19-20 crit range (flavored as just dealing more damage due to the necrotic damage), or it could apply the next save made at disadvantage debuff.

It would be more easy to exploit if one of the characters is a Champion, but I don't think that's a real issue. 

Another idea would be that the creature has vulnerability to the weapon damage of the next weapon attack or the spell damage of the next spell that damages it. That idea would have to be fleshed out better and the idea would be that the vulnerability wouldn't include any additional sources of damage like sneak attack or divine smite. It probably wouldn't work well or the mechanics wouldn't work with the thematics, so it's probably a busted idea. 

I'm not sure about this. I think that would be too powerful, and I think the current versions have been mostly fixed. 

Okay, I have created some ranged versions of blade cantrips in this thread, called Elemental Shot Cantrips. I would appreciate feedback if you have any.