Would it be balanced to introduce an "Aim Action" that uses the same mechanics as True Strike?

Question

There are few things in D&D 5E on which the consensus is as overwhelming as on the fact that True Strike is an extremely poor cantrip. As discussed in many places, the main problem is the action economy. As this answer to a similar question puts it, casting True Strike is not so much a benefit as it is a trade off: You waste your action on one turn in order to gain advantage and thus use your action more effectively on your next turn. As has also been pointed out countless times, this is rarely optimal, as attacking twice without advantage still has the chance of hitting twice while the chance of hitting at least once is the same as when attacking once with advantage.

The generic scenario in which True Strike actually helps is when a character needs to put all their focus into making sure that one crucial, strategically relevant attack actually hits. Putting it like that doesn't let it sound particularly magical: the character just takes their time to aim. I really like the narrative and I could imagine very dramatic, epic, and satisfying game moments with this mechanic. However, they do not happen, since they alone are hardly a convincing reason to choose the otherwise almost useless True Strike as one of one's limited cantrips.

Would it be balanced to remove True Strike from the game entirely and introduce an Aim Action that is available to everyone and does exactly what True Strike does (mechanically)?

Because this Aim Action is a trade off that is suboptimal most of the time, I would consider it balanced for my game, unless one of the following questions has a positive answer:

Would it step on anyone's toes by making certain class features or spells (other than True Strike of course) obsolete? Are there any exploits in other scenarios than the one described above that I am overlooking?

---

A sidenote: I also noticed that the Unearthed Arcana Class Feature Variants include the "Cunning Action: Aim" for the rogue.

You gain an additional way to use your Cunning Action: carefully aiming your next attack. As a bonus action, you give yourself advantage on your next attack roll on the current turn. You can use this bonus action only if you haven’t moved during this turn, and after you use the bonus action, your speed is 0 until the end of the current turn.

Having a general Aim Action and allowing this feature variant would keep in line with the idea that the Cunning Action lets the rogue do things anyone can do, just quicker. Of course, one could say that 1.) it's Unearthed Arcana, so it's not a reference for balance, and 2.) the relation between Aim and Cunning Action: Aim is not as simple as with the uses of the Cunning Action from the PHB, but I still thought this observation could be worth mentioning.

Answer 1

You don't need to do this, because everyone can already do this

The GM is supposed to confer advantage/disadvantage rather than apply modifiers whenever circumstances are altered to give one combatant an edge over another:

The GM can also decide that circumstances influence a roll in one direction or the other and grant advantage or impose disadvantage as a result.

(also c.f. this forum post)

Having taken a round to line up a shot should definitely count as circumstances beneficial to the attacker (that's literally why people aim), so the attacker should already be being given advantage without any house-ruling necessary.

This isn't unbalanced, either: the 'Help' action already lets anyone (or their pet mouse) grant advantage with an action on most things, including attack rolls on targets within 5 feet, and circumstances where an action's worth of work can't modify circumstances in some other way sufficient to grant advantage are extremely rare. Trading an action for advantage is pretty much going to be possible all the time anyways, so letting people do it via 'aiming' really isn't a problem.

Answer 2

Yes, but it would need a higher cost than one action

5e is pretty heavily based on building advantage in different ways, and the trade off to gaining that advantage. There are two examples of advantage and there costs that I feel relevant, and I think we can find an appropriate cost between them:

• True Strike (PHB p.284): the current costs for true strike are: A cantrip slot used for this spell, an action, and concentration. This spell can only be cast on a target within 30 feet of the caster.

This is pretty well agreed upon as being too high of a cost for a single advantage on one attack roll (thus you posing this question). This might work for an Aim action, however, as that would take away the cantrip slot use. Making this action available for all creatures would probably require one more thing to balance this action out.

• Shoving a Creature Prone (PHB p.195): Using the Attack action, you can make a Special melee Attack to shove a creature, either to knock it prone or push it away from you. If you’re able to make multiple attacks with the Attack action, this Attack replaces one of them. The target must be no more than one size larger than you and must be within your reach. Instead of Making an Attack roll, you make a Strength (Athletics) check contested by the target’s Strength (Athletics) or Dexterity (Acrobatics) check (the target chooses the ability to use). If you win the contest, you either knock the target prone or push it 5 feet away from you.

What I would think is relevant for an Aim action would be the contested roll. Probably the Aimer's Insight (Wisdom) against the target's Deception (Charisma). Combining the relevant parts of these two methods of gaining advantage, I've come to this result:

Aim

You take a moment to study a creature, taking care to line up your next attack. As an action, choose a target within 30 feet. Make an contested Insight (Wisdom) check against the targets Deception (Charisma). If you win the contest, your next attack roll before the end of your next turn is made with advantage against the target. You must maintain concentration on aiming, as though concentrating on a spell.