The arcade games don't count. You wanna know why? 'Cuz they suck. I played both of them. The controls for jumping and drifting were bullshit, and the items SUCKED. Each character got their own lame item. I don't even remember what they were they were so bad. It was like... shoot a ball of energy! kind of crap. The only redeeming thing about the game is the camera that takes your photo and puts it on your character on the radar and whatnot. So you could make A TOTALLY WACKY FACE for ultimate lulz, which I did every time.
Namco went buckwild with those games. I recommend trying them once or twice if you find them, but then moving on with your life.
Diddy Kong is the "spiritual successor" to Donkey Kong Jr. Mario fought Donkey Kong in Donkey Kong, and then he fought Donkey Kong Jr. (and lost) in Donkey Kong Jr. Some say that Donkey Kong became Cranky Kong and that Donkey Kong Jr. became today's "king of swing" DK, and Diddy Kong is basically the new Donkey Kong Jr. I say it works.
Concerning R.O.B.: Nintendo has been trying to give R.O.B. the big comeback he deserves for a long time. One of the stages in F-Zero GX had you racing around a gigantic kickass R.O.B., and then R.O.B. was in MKDS, and now R.O.B.'s in Brawl. He's back and he's back big time, and there ain't no complaining. This is R.O.B. people, he's a ROBOTIC OPERATING BUDDY. You can't mess with that. He belongs in every game ever.
If Samus was in Mario Kart, then Link would have to be in Mario Kart, since they both had their cameos in SMRPG, if that's what you're going by. But once you've opened the doors to those 2 universes, we'd need a Zelda and a Ganondorf. This isn't Mario Kart Melee here!