What is the tradeoff?
Discussions of balancing new homebrew content are regularly confusing their goals. When introducing new content, you should consider what effect it has on content that already exists, and gameplay that will arise from its inclusion.
One big aspect of that is whether or not it dominates over an additional choice already in the game. For example, recovering from the unconscious status associated with being at 0 HP and stable requires that a character:
- Receive healing from a potion, spell, or other direct source.
- Use at least one Hit Dice during a Short Rest of at least 1 hour in length.
- Regain 1 hit point after being stable for 1d4 hours.
This Revive Ally spell seems to be competing with Short Rest what it brings to the table.
- Requires 1 hour of time.
- Requires a player resource (at least one Hit Die)
- Requires 5 minutes of time.
- Occupies a Class Feature (cantrip selection).
- Has no resource requirement.
So what Revive Ally equates to in terms of its tradeoff is that in exchange for one choice Cantrip on one of the characters, you are allowing that player to spend 5 minutes to elevate Stable characters to 1 HP, even if they are out of Hit Dice.
I would argue that this is a passable tradeoff. The Stable character won't get healed up enough to withstand any amount of damage and remain conscious, but in exchange they don't need to wait a full hour of game time before they're able to take action.
In my games, if players have been taken down and the party is short on resources to spend on bringing everyone back up, then the party is expected to be more cautious and careful with their unconscious friends until they have time to recover. Players are expected to act in their best interests, to keep themselves healthy and avoid brash decisions that would put them or their friends in unneeded jeopardy. If the game you want to run has no need for this element of tension to be present, then there is no issue.
What is this solving? (What is the real problem?)
On the surface, the purpose behind this cantrip is a reaction to a common occurrence where a party member is down but stable and the other party members are unable to, or are refusing to, use resources to bring the downed member up.
While this cantrip would be a mechanism by which characters can reliably bring a party member up from 0 without a recurring resource cost, this seems to be hiding a different problem.
After a nearly fatal combat has taken part, the party doesn't have time to spend on a Short Rest. It's very likely that many parts of a campaign may be more time critical than others, and a Short Rest is not always available at the drop of a hat. However, the party seems unprepared for this time pressure in one way or another. The pacing of the campaign may be a bit too fast for the party, the plot driving players to expend most of their resources before the day's end, emptying their strategic reserves. An alternative solution is to tone down combat encounters to reduce party resource expenditure or increasing the rate of rewards that include Healing Potions.
Alternatively, it could be that narrative focus is spent on this time when some players are incapacitated. Maybe the party has just acquired an artifact of some sort and the conscious party members wants to engage with it immediately. This is an opportunity to draw with broader strokes, so to speak. Allow the party to take a short rest, during which the downed players will definitely be recovering, but loosen the flow of time in a way that allows the downed players to act during the rest as they recover.
"Hey..." Abraham gruelingly rights himself up and sits against the dungeon wall, barely holding onto life. "Let me see those inscriptions..."
Should you add this to your game?
Ultimately, this is on you to decide. Balance is meaningless in the face of a DM wanting to include or exclude content. What matters more is that you understand what these new rogue elements are going to introduce to the activity.
Even perfectly balanced content might serve no better purpose being added to the game. If no one uses it because the tradeoff it introduces doesn't give enough of a benefit over existing options, or it's not otherwise appetizing to players, a perfectly fair homebrew spell or class may as well not even be added.
I hope I've provided helpful insight into the problem you're trying to solve with this cantrip, and I hope this answer helps you decide if you're going to add it to your game.