It's annoying that Objective-C doesn't let you provide a default implementation for a protocol. Maybe one could just create a proxy with that?

forwarding_for_default_protocol_implementations.m

//
//  main.m
//  ProtocolImplementations
//
//  Created by Uli Kusterer on 07/09/15.
//  Copyright (c) 2015 Uli Kusterer. All rights reserved.
//

#import <Foundation/Foundation.h>

@interface PISBaseClass : NSObject

-(void) doBaseClassStuff;

@end


@implementation PISBaseClass

-(void) doBaseClassStuff
{
    NSLog(@"%@", NSStringFromSelector(_cmd) );
}

@end


@protocol PISMixInProtocol <NSObject>

@optional   // Sadly needed or PISSubClass will complain about missing impl.
-(void) doMixInStuff;

@end


@interface PISMixInShim : NSObject <PISMixInProtocol>

-(instancetype)   initWithBaseClass: (PISBaseClass*)bc;

-(void) doMixInStuff;

@end


@interface PISMixInShim ()
{
    PISBaseClass*   baseClassObj;
}
@end

@implementation PISMixInShim

-(instancetype)   initWithBaseClass: (PISBaseClass*)bc
{
    if( !bc )
        return nil;
    
    self = [super init];
    if( self )
    {
        baseClassObj = bc;
    }
    return self;
}


-(void) doMixInStuff
{
    NSLog(@"%@", NSStringFromSelector(_cmd) );
}


-(BOOL) respondsToSelector:(SEL)aSelector
{
    if( ![super respondsToSelector: aSelector] )
    {
        return [baseClassObj respondsToSelector: aSelector];
    }
    return YES;
}


-(NSMethodSignature *)  methodSignatureForSelector:(SEL)aSelector
{
    NSMethodSignature *sig = [super methodSignatureForSelector:aSelector];
    if( !sig )
        sig = [baseClassObj methodSignatureForSelector:aSelector];
    return sig;
}

-(void) forwardInvocation:(NSInvocation *)anInvocation
{
    [anInvocation invokeWithTarget: baseClassObj];
}

@end



@interface PISSubClass : PISBaseClass <PISMixInProtocol>

-(instancetype)   init;

-(void) doSubClassStuff;

@end


@implementation PISSubClass

-(instancetype)   init
{
    self = [super init];
    if( self )
    {
    
    }
    return (id)[[PISMixInShim alloc] initWithBaseClass: self];
}


-(void) doSubClassStuff
{
    NSLog(@"%@", NSStringFromSelector(_cmd) );
}

@end


int main(int argc, const char * argv[]) {
    @autoreleasepool {
        PISSubClass *   sco = [[PISSubClass alloc] init];
        
        [sco doBaseClassStuff];
        [sco doSubClassStuff];
        [sco doMixInStuff];
    }
    return 0;
}