Here we go...
Let's start with popularity. It seems as though Diddy Kong is a popularity just looking at the amount of pages in this topic). Diddy kong is a popular character from the series that we all know and love. I would love the idea to see him in there, and many people want to because he is a popular character, and a fun character at that.
Now, take a look at the amount of Donkey Kong characters in the game, and it resolves in 1. For such a popular franchise, I would love to see another one come into the series, and bring something new to the game because Diddy dosen't look like he will be a clone. This is a great way to make the franchise larger, and bring it to newer hights. He has stared in a variety of games, and has a cool personality, and may add something we would love to the game.
As for a mental picture, I can actually see him making it into the game, and I think I could see him fighting. From that, movesets are created, and the character comes to jubilant life. Soon, we think of a character who will probably be great for the game, and then we think about how he will seperate himself from DK. But, he really can be much different from DK and establish himself as a unique character. It's like Zelda and Sheik, i am not suggesting they should be together, but they are the same person (Not suggesting that DK and Diddy are) and they have been completely different characters. So, I think the oppuirtunities are open for this character, and i hope Diddy make the cut for Brawl.
-SMB