I'm kind of late here, but I'll throw my answer out there. The only part that really matters is 'At a creature'. There's no definition for this anywhere in the rules, so we'll have to abide by the Good Faith reading of Potent Cantrip itself to decide what meaning that phrase has, and how it applies to the old weapon cantrips.
We can divide Potent Cantrip into parts, and deal with each one seprately.
- 'When you cast a cantrip' -- Fairly self-explanatory, and all of the cantrip listed above are, indeed, cantrips.
- 'At a creature' -- The point at issue, and we'll have to read the rest of the rule in good faith to define this properly.
- 'And you miss with the attack roll or the target succeeds on a saving throw against the cantrip' -- All of the cantrips listed above (except Shilleleagh, which is obviously excluded) make an attack roll, so, at first glance, they all meet the qualifications. Furthermore, is making an attack roll against a creature or forcing a creature to make a saving throw covered in any rule? Yes, and that rule is the 'Target' rule. It reads as follows: "A target is the creature or object targeted by an attack roll, forced to make a saving throw by an effect, or selected to receive the effects of a spell or another phenomenon." Thus, using the good faith rule, we can conclude that the 'At a creature' segment of the Potent Cantrip ability is a modification of the 'Target' rule that requires us to target a creature instead of either a creature or an object.
Another part of the initial question is what do we do about cantrips that have a range of 'Self'? First, what is Range?
From the spellcasting rules: A spell’s range indicates how far from the spellcaster the spell’s effect can originate, and the spell’s description specifies which part of the effect is limited by the range.
A range usually takes one of the following forms:
Distance. The range is expressed in feet.
Touch. The spell’s effect originates on something the spellcaster must touch, as defined in the spell.
Self. The spell is cast on the spellcaster or emanates from them, as specified in the spell.
If a spell has movable effects, they aren’t restricted by its range unless the spell’s description says otherwise.
We then have to ask whether a spell with a Range of Self is targetting the caster. The Target rule allows a target to be a creature selected to recieve the effects of a spell. Does the caster of a spell with a Range of Self apply the effects of the spell to themselves as a target? That depends on the effect applied, according to the Effects rule of the Spellcasting rules, which reads (in part) as follows:
The effects of a spell are detailed after its duration entry. Those details present exactly what the spell does, which ignores mundane physical laws; any outcomes beyond those effects are under the DM’s purview. Whatever the effects, they typically deal with targets, saving throws, attack rolls, or all three, each of which is detailed below.
Targets
A typical spell requires the caster to pick one or more targets to be affected by the spell’s magic. A spell’s description says whether the spell targets creatures, objects, or something else.
A Clear Path to the Target. To target something with a spell, a caster must have a clear path to it, so it can’t be behind Total Cover.
Targeting Yourself. If a spell targets a creature of your choice, you can choose yourself unless the creature must be Hostile or specifically a creature other than you.
Areas of Effect. Some spells, such as Thunderwave, cover an area called an area of effect, which is defined in the rules glossary. The area determines what the spell targets. The description of a spell specifies whether it has an area of effect, which is typically one of these shapes: Cone, Cube, Cylinder, Emanation, Line, or Sphere.
Thus, by a good faith reading of the rules, Range tells you only the possible distance at which a spell effect can be applied, and does not tell you what can be the target or targets of the spell's effect, and the Effect rule specifies that the spell description defines the possible target or targets of the spells effect. Thus, in good faith, Range does not determine the target of the spell, the spell effect determines the target of the spell. Thus, Booming Blade, Greenflame Blade, and True Strike, all of which have a Range of Self, have an effect which specifies what the spell can target. Both Booming Blade and Green-Flame Blade have a Range of Self (5 feet), which the spell description qualifies as the range at which you can make a melee attack with the weapon used in the spell's casting.
At this point, I think we've established the following: Any cantrip, regardless of the specified Range, that allows you to select a target or targets as a part of the effect, and permits an attack roll against that target or targets, or forces the target or targets to make a saving throw, meets the qualifications of the first three parts of the Potent Cantrip ability, so long as you miss the attack roll or rolls, or the target or targets succeed on their saving throw.
- 'The target takes half the cantrip's damage (if any) but suffers no additional effect from the cantrip' -- this last point is fairly simple: if you would deal damage as a result of hitting with an attack roll or as a result of a target failing a saving throw, you may deal half of this damage if you miss an attack roll or the target succeeds on their saving throw.
