Instead of giving advantage... just make the next attack against the creature you target hit at the cost of a potential crit. You cast it and the next person to attack the target just rolls damage.
You don't even have to bump it up to a first level spell... even characters that can cast a cantrip on a bonus action are still sacrificing a bonus action. There are some really good things you could do on a bonus action.
Action economy is so expensive that this rule change is actually incredibly balanced when compared to other actions and spells. Just think of it this way, unless you can cast it on a bonus action, you could be potentially sacrificing three attacks to have one hit for sure. The cost of the spell is built into this version.
Alternative option:
Have it include an attack that just hits - but make it a save. The more I think about this spell, the more I realize that it really one of the worst spells ever. These changes would at least make it mid-level good.
That's making it awfully strong to not have an attack roll at all. But ironically, less good if the next attacker is a Rogue, since they wouldn't have the benefit of attacking with Advantage to get Sneak Attack if there isn't an enemy of the target within 5 feet.
Maybe you could have it work by giving advantage on the target to the next ally within 30 feet of you, since the point of the cantrip is to gain insight about the target's weak point, which you would then communicate to someone nearby. Sort of like a longer-distance help action but with limitations. (Also you'd still need to Concentrate.)
It's actually not that strong. You can't crit from it. You are sacrificing a whole turn to do this unless you are a very specific class (Eldritch Knight) that can cast cantrips on a bonus action. And even then... still not that strong because it only affects one attack. It would be great for rogues, but they can't cast a cantrip on a bonus action.
In other words, it still takes up a whole action, which when you have multiple attacks, it's statistically not better. The usefulness is questionable at best the moment you have multiple attacks.
In terms of power, I'd give it a 4 out of 10.
For context, Fireball at level 5 I consider like a 9.5 out of 10. Note: That is only at level 5, the power still stays great for most of the game but eventually starts dropping off later.
If you are going to claim that this change is too strong, I'd like a valid argument of how that would break the game with details please. Otherwise do not claim that.
Here's my breakdown of how this wouldn't break the game.
Wizard casts an action, true strike. Barbarian runs up and rolls damage for his first attack. Gets no benefits from his powers that increase his crit chance. Uses reckless attack on second attack, has to roll normally. Now without the spell. Half-Orc Barbarian runs up and rolls a 19, that's a crit. Gets triple dice damage. Rolls again and hits.
Which one does more damage? Obviously the one where he crits on a 19. The likelihood of that happening is 10%. The likelihood of him hitting at all with a 20 strength is something like 60-75% typically with reckless. He sacrifices that 10% on a crit, to gain 100% likelihood on a hit.
It's a statistical trade-off and it costs an action. Now this could cause some issues if the range of the spell was more than 30 feet. In which case you could probably abuse this but that would require SEVERAL feats and I don't think you can choose true strike from the spell sniper list. A sorcerer could extend the range, but only to 60 feet.
Here, I'll even throw you a bone and say this spell can be blocked by shield. Now it's like magic missile but it does no damage.
I think you are mistaken when you say this is an awfully strong change. It is not awfully strong, it's only slightly better.
After playing a Rogue recently, I'm picturing the Rogue hidden behind the BBEG, poised to strike and deal some Sneak Attack damage with their clear advantage on the target and a +1 weapon. But then the Sorcerer goes first and casts Insta-True Strike. No other party members are near the BBEG. Now the Rogue could just roll damage, but would not get any Sneak Attack. So all because of the Sorcerer "helping", the Rogue needs to wait until they can attack "for real" in order to deal the real damage a Rogue is meant to dish out.
Perhaps I'm thinking about how this has more potential to mess with your allies' attack abilities than confer useful benefit, which could make the caster a lot less popular with their party even if this cantrip is technically better than regular True Strike. Either give just the caster the benefit of insta-hit on their next turn, or else give advantage to other attackers.
Instead of giving advantage... just make the next attack against the creature you target hit at the cost of a potential crit. You cast it and the next person to attack the target just rolls damage.
You don't even have to bump it up to a first level spell... even characters that can cast a cantrip on a bonus action are still sacrificing a bonus action. There are some really good things you could do on a bonus action.
Action economy is so expensive that this rule change is actually incredibly balanced when compared to other actions and spells. Just think of it this way, unless you can cast it on a bonus action, you could be potentially sacrificing three attacks to have one hit for sure. The cost of the spell is built into this version.
Alternative option:
Have it include an attack that just hits - but make it a save. The more I think about this spell, the more I realize that it really one of the worst spells ever. These changes would at least make it mid-level good.
That's making it awfully strong to not have an attack roll at all. But ironically, less good if the next attacker is a Rogue, since they wouldn't have the benefit of attacking with Advantage to get Sneak Attack if there isn't an enemy of the target within 5 feet.
Maybe you could have it work by giving advantage on the target to the next ally within 30 feet of you, since the point of the cantrip is to gain insight about the target's weak point, which you would then communicate to someone nearby. Sort of like a longer-distance help action but with limitations. (Also you'd still need to Concentrate.)
Helpful rewriter of Japanese->English translation and delver into software codebases (she/e/they)
It's actually not that strong. You can't crit from it. You are sacrificing a whole turn to do this unless you are a very specific class (Eldritch Knight) that can cast cantrips on a bonus action. And even then... still not that strong because it only affects one attack. It would be great for rogues, but they can't cast a cantrip on a bonus action.
In other words, it still takes up a whole action, which when you have multiple attacks, it's statistically not better. The usefulness is questionable at best the moment you have multiple attacks.
In terms of power, I'd give it a 4 out of 10.
For context, Fireball at level 5 I consider like a 9.5 out of 10. Note: That is only at level 5, the power still stays great for most of the game but eventually starts dropping off later.
If you are going to claim that this change is too strong, I'd like a valid argument of how that would break the game with details please. Otherwise do not claim that.
Here's my breakdown of how this wouldn't break the game.
Wizard casts an action, true strike. Barbarian runs up and rolls damage for his first attack. Gets no benefits from his powers that increase his crit chance. Uses reckless attack on second attack, has to roll normally. Now without the spell. Half-Orc Barbarian runs up and rolls a 19, that's a crit. Gets triple dice damage. Rolls again and hits.
Which one does more damage? Obviously the one where he crits on a 19. The likelihood of that happening is 10%. The likelihood of him hitting at all with a 20 strength is something like 60-75% typically with reckless. He sacrifices that 10% on a crit, to gain 100% likelihood on a hit.
It's a statistical trade-off and it costs an action. Now this could cause some issues if the range of the spell was more than 30 feet. In which case you could probably abuse this but that would require SEVERAL feats and I don't think you can choose true strike from the spell sniper list. A sorcerer could extend the range, but only to 60 feet.
Here, I'll even throw you a bone and say this spell can be blocked by shield. Now it's like magic missile but it does no damage.
I think you are mistaken when you say this is an awfully strong change. It is not awfully strong, it's only slightly better.
After playing a Rogue recently, I'm picturing the Rogue hidden behind the BBEG, poised to strike and deal some Sneak Attack damage with their clear advantage on the target and a +1 weapon. But then the Sorcerer goes first and casts Insta-True Strike. No other party members are near the BBEG. Now the Rogue could just roll damage, but would not get any Sneak Attack. So all because of the Sorcerer "helping", the Rogue needs to wait until they can attack "for real" in order to deal the real damage a Rogue is meant to dish out.
Perhaps I'm thinking about how this has more potential to mess with your allies' attack abilities than confer useful benefit, which could make the caster a lot less popular with their party even if this cantrip is technically better than regular True Strike. Either give just the caster the benefit of insta-hit on their next turn, or else give advantage to other attackers.
Helpful rewriter of Japanese->English translation and delver into software codebases (she/e/they)