Xanathar's Guide to Everything (p. 85) gives an optional rule if a character wants to identify a spell:
Sometimes a character wants to identify a spell that someone else is casting or that was already cast. To do so, a character can use their reaction to identify a spell as it's being cast, or they can use an action on their turn to identify a spell by its effect after it is cast.
If the character perceived the casting, the spell's effect, or both, the character can make an Intelligence (Arcana) check with the reaction or action. The DC equals 15 + the spell's level. If the spell is cast as a class spell and the character is a member of that class, the check is made with advantage. For example, if the spellcaster casts a spell as a cleric, another cleric has advantage on the check to identify the spell. Some spells aren't associated with any class when they're cast, such as when a monster uses its Innate Spellcasting trait.
I'm thinking of completely replacing the check with comparing the passive Arcana score and the DC (15 + spell level). This would also not require a reaction to perform.
However, I'm not sure whether this will be balanced or not. The goal is to allow identifying spell before casting counterspell. The side goal is to reduce dice rolling.
One of my main concern is early level this would be useless, as even with proficiency in Arcana, you would only get at most +5 (+3 from INT and +2 from proficiency). Except if you have expertise, you will never be able to identify even 1st level spell. Without expertise, you will only be able to identify up to 5th level (10 +5 INT and +5 proficiency against DC 20 for 5th level spell).
Is this modification (completely replacing identification using passive score instead of actively rolling Arcana) underpowered? Can it be remedied by changing the base DC to 10 instead of 15? What are some other concern I might have missed?
This rule will also apply on other situations other than for counterspell.