NSArray thought experiment. This code may not compile and there may be subtle bugs/inconsistencies. But this serves to illustrate the possible solution outlined in https://www.dzombak.com/blog/2015/05/Cocoa-s-mutable-subclass-pattern-is-an-antipattern.html .

NSArray.h

@protocol NSArray

+ (instancetype)array;
+ (instancetype)arrayWithObject:(id)anObject;
+ (instancetype)arrayWithObjects:(const id [])objects count:(NSUInteger)cnt;
+ (instancetype)arrayWithObjects:(id)firstObj, ... NS_REQUIRES_NIL_TERMINATION;
+ (instancetype)arrayWithArray:(NSArray *)array;

- (instancetype)initWithObjects:(id)firstObj, ... NS_REQUIRES_NIL_TERMINATION;
- (instancetype)initWithArray:(NSArray *)array;
- (instancetype)initWithArray:(NSArray *)array copyItems:(BOOL)flag;

- (instancetype)init NS_DESIGNATED_INITIALIZER;
- (instancetype)initWithObjects:(const id [])objects count:(NSUInteger)cnt NS_DESIGNATED_INITIALIZER;
- (instancetype)initWithCoder:(NSCoder *)aDecoder NS_DESIGNATED_INITIALIZER;

@property (readonly) NSUInteger count;

- (id)objectAtIndex:(NSUInteger)index;

@property (nonatomic, readonly) id firstObject NS_AVAILABLE(10_6, 4_0);
@property (nonatomic, readonly) id lastObject;

- (instancetype)arrayByAddingObject:(id)anObject;
- (instancetype)arrayByAddingObjectsFromArray:(NSArray *)otherArray;

- (NSString *)componentsJoinedByString:(NSString *)separator;

- (BOOL)containsObject:(id)anObject;
- (id)firstObjectCommonWithArray:(NSArray *)otherArray;
- (void)getObjects:(id __unsafe_unretained [])objects range:(NSRange)range;

- (instancetype)subarrayWithRange:(NSRange)range;

- (NSUInteger)indexOfObject:(id)anObject;
- (NSUInteger)indexOfObject:(id)anObject inRange:(NSRange)range;
- (NSUInteger)indexOfObjectIdenticalTo:(id)anObject;
- (NSUInteger)indexOfObjectIdenticalTo:(id)anObject inRange:(NSRange)range;

- (NSUInteger)indexOfObjectPassingTest:(BOOL (^)(id obj, NSUInteger idx, BOOL *stop))predicate NS_AVAILABLE(10_6, 4_0);
- (NSUInteger)indexOfObjectWithOptions:(NSEnumerationOptions)opts passingTest:(BOOL (^)(id obj, NSUInteger idx, BOOL *stop))predicate NS_AVAILABLE(10_6, 4_0);
- (NSUInteger)indexOfObjectAtIndexes:(NSIndexSet *)s options:(NSEnumerationOptions)opts passingTest:(BOOL (^)(id obj, NSUInteger idx, BOOL *stop))predicate NS_AVAILABLE(10_6, 4_0);

- (NSIndexSet *)indexesOfObjectsPassingTest:(BOOL (^)(id obj, NSUInteger idx, BOOL *stop))predicate NS_AVAILABLE(10_6, 4_0);
- (NSIndexSet *)indexesOfObjectsWithOptions:(NSEnumerationOptions)opts passingTest:(BOOL (^)(id obj, NSUInteger idx, BOOL *stop))predicate NS_AVAILABLE(10_6, 4_0);
- (NSIndexSet *)indexesOfObjectsAtIndexes:(NSIndexSet *)s options:(NSEnumerationOptions)opts passingTest:(BOOL (^)(id obj, NSUInteger idx, BOOL *stop))predicate NS_AVAILABLE(10_6, 4_0);

@property (readonly, copy) NSString *description;
- (NSString *)descriptionWithLocale:(id)locale;
- (NSString *)descriptionWithLocale:(id)locale indent:(NSUInteger)level;

@property (readonly, copy) NSData *sortedArrayHint;

- (instancetype)sortedArrayUsingFunction:(NSInteger (*)(id, id, void *))comparator context:(void *)context;
- (instancetype)sortedArrayUsingFunction:(NSInteger (*)(id, id, void *))comparator context:(void *)context hint:(NSData *)hint;
- (instancetype)sortedArrayUsingSelector:(SEL)comparator;

- (instancetype)sortedArrayUsingComparator:(NSComparator)cmptr NS_AVAILABLE(10_6, 4_0);
- (instancetype)sortedArrayWithOptions:(NSSortOptions)opts usingComparator:(NSComparator)cmptr NS_AVAILABLE(10_6, 4_0);

@end


@interface NSArray : NSObject <NSCopying, NSSecureCoding, NSFastEnumeration>

+ (instancetype)arrayWithContentsOfFile:(NSString *)path;
+ (instancetype)arrayWithContentsOfURL:(NSURL *)url;
- (instancetype)initWithContentsOfFile:(NSString *)path;
- (instancetype)initWithContentsOfURL:(NSURL *)url;

- (NSEnumerator *)objectEnumerator;
- (NSEnumerator *)reverseObjectEnumerator;

 (void)enumerateObjectsUsingBlock:(void (^)(id obj, NSUInteger idx, BOOL *stop))block NS_AVAILABLE(10_6, 4_0);
- (void)enumerateObjectsWithOptions:(NSEnumerationOptions)opts usingBlock:(void (^)(id obj, NSUInteger idx, BOOL *stop))block NS_AVAILABLE(10_6, 4_0);
- (void)enumerateObjectsAtIndexes:(NSIndexSet *)s options:(NSEnumerationOptions)opts usingBlock:(void (^)(id obj, NSUInteger idx, BOOL *stop))block NS_AVAILABLE(10_6, 4_0);

- (BOOL)writeToFile:(NSString *)path atomically:(BOOL)useAuxiliaryFile;
- (BOOL)writeToURL:(NSURL *)url atomically:(BOOL)atomically;

@end

NSMutableArray.h

@interface NSMutableArray : NSObject <NSMutableCopying>

+ (instancetype)arrayWithCapacity:(NSUInteger)numItems;

- (instancetype)init NS_DESIGNATED_INITIALIZER;
- (instancetype)initWithCapacity:(NSUInteger)numItems NS_DESIGNATED_INITIALIZER;

- (void)addObject:(id)anObject;
- (void)insertObject:(id)anObject atIndex:(NSUInteger)index;
- (void)removeLastObject;
- (void)removeObjectAtIndex:(NSUInteger)index;
- (void)replaceObjectAtIndex:(NSUInteger)index withObject:(id)anObject;

- (void)addObjectsFromArray:(NSArray *)otherArray;
- (void)exchangeObjectAtIndex:(NSUInteger)idx1 withObjectAtIndex:(NSUInteger)idx2;
- (void)removeAllObjects;
- (void)removeObject:(id)anObject inRange:(NSRange)range;
- (void)removeObject:(id)anObject;
- (void)removeObjectIdenticalTo:(id)anObject inRange:(NSRange)range;
- (void)removeObjectIdenticalTo:(id)anObject;
- (void)removeObjectsFromIndices:(NSUInteger *)indices numIndices:(NSUInteger)cnt NS_DEPRECATED(10_0, 10_6, 2_0, 4_0);
- (void)removeObjectsInArray:(NSArray *)otherArray;
- (void)removeObjectsInRange:(NSRange)range;
- (void)replaceObjectsInRange:(NSRange)range withObjectsFromArray:(NSArray *)otherArray range:(NSRange)otherRange;
- (void)replaceObjectsInRange:(NSRange)range withObjectsFromArray:(NSArray *)otherArray;
- (void)setArray:(NSArray *)otherArray;
- (void)sortUsingFunction:(NSInteger (*)(id, id, void *))compare context:(void *)context;
- (void)sortUsingSelector:(SEL)comparator;

- (void)insertObjects:(NSArray *)objects atIndexes:(NSIndexSet *)indexes;
- (void)removeObjectsAtIndexes:(NSIndexSet *)indexes;
- (void)replaceObjectsAtIndexes:(NSIndexSet *)indexes withObjects:(NSArray *)objects;

- (void)setObject:(id)obj atIndexedSubscript:(NSUInteger)idx NS_AVAILABLE(10_8, 6_0);

- (void)sortUsingComparator:(NSComparator)cmptr NS_AVAILABLE(10_6, 4_0);
- (void)sortWithOptions:(NSSortOptions)opts usingComparator:(NSComparator)cmptr NS_AVAILABLE(10_6, 4_0);

@end