grammar - In "Enter John", is John in the nominative or accusative case?

This question made me think about the structure of the sentence.

I'm familiar with the expression 'Enter Michael'/'Exit John' to represent Michael's or John's entry or exit, respectively, to a dramatic stage, and its metaphorical use.

Is this a real sentence, or a sentence fragment? Either way, should John be treated as the subject of the sentence, and put in the nominative case? I realise that this makes little practical difference as English words are not typically declined based on case, and "exit him" or "Enter I" sounds wrong either way. However I am curious.

Initially I had thought that "exit" might be the subjunctive, and that the phrase was equivalent to "Let John exit". But the plural of "exit" in stage directions is "exeunt", and both "exit" and "exeunt" are present indicative in Latin.

Answer

My interpretation is that we are looking at a nominative. Here is why.
The key to the noun case lies I believe in the analysis of the verb inflection.

One first need to acknowledge that scene directions in play-texts used to take various forms¹.

1. All Latin:
   
   
   
   • Present indicative mood: intrat/intrant and exit/exeunt. In which case the characters are in nominative case.
   
   
   • Present subjective mood: exeat/exeant (nominative as well)
   
   
   For instance in exeunt omnes as you note, exeunt is the 3rd person plural of the present indicative of the verb exeo.
   In theory, omnes is possibly the masculine plural nominative, vocative or accusative of omnis.
   Since exeunt is not an imperative, we can rule out vocative.
   Since exeunt is a plural we can rule out accusative - if it were an accusative we would have exit omnes, not exeunt omnes, some kind of "there exit all".


2. Mixed English/Latin:
   
   
   • Present indicative mood: Entreth[enters]/enter (singular plural) and exit/exeunt. For instance "Enters the King". The King is a nominative case (note that the verb is inflected as a 3rd person singular).
   
   
   • If we read Enter John, one could interpret enter as an imperative in which case John is a vocative. but that doesn't square up with exit (a Latin indicative - the imperative would be exi or exito) and seems awkward in the case of "Enter the King".
     There is another explanation though. I would venture that since exit is both a Latin 3rd person singular and an English infinitive, the enter form is a symmetric infinitive. Said otherwise, the exit started as a Latin indicative and was retained because it looked like an English infinitive. This would explain the weird mix of English and Latin (enter is no Latin word of course). In this case, the noun is in the nominative case.
     
   
   
   
   

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Note 1
A Japanese professor of European literature named Mariko Ichikawa has actually devoted a (freely available) whole article to Shakespearean Entrances. Here is a short excerpt:

A survey of stage directions in early English renaissance plays shows that the use of the English direction 'Enter' (presented in the imperative mood and used for either one or more characters) along with the Latin direction 'Exit'/'Exeunt' (in the present indicative mood) was not completely established until the early or mid-1590s. Some of the earliest surviving play-texts basically employ the convention of massing the names of all the characters who appear in any given scene in one list at its opening, without the word 'enter' or its equivalent, in imitation of the printed form of Roman comedy current in the Renaissance. Some almost exclusively use the Latin directions: 'intrat'/'intrant' (in the present indicative mood) and either 'exeat'/'exeant' (in the present subjective mood) or 'exit'/'exeunt'. Others normally use vernacular verbs such as 'come in' and 'go out', and 'enter' and 'go out'. 'Entreth[enters]'/'enter' (singular plural) and 'exit'/'exeunt' are usually found[...]