So just because Nintendo buys SEGA, suddenly Sonic gets as many characters as freaking Pokemon? That absolutely would not happen.
Sonic has sold a third of what Pokemon has solely off of Nintendo platforms, and he's existed for six years longer.
There aren't even that many noteworthy Sonic characters to begin with. I love Sonic, but hate the majority of his friends.
Their relationship with Nintendo being so good right now, I could see a buyout/merger being possible. They definitely won't get as many characters as Pokemon, hell, even Mario doesn't have as many, but I have no doubts in my mind that if Sega became a part of Nintendo they would get more characters. They could easily have 3 or 4 but they would most likely have to be originals (by originals, I mean debuted on the Genesis.) It could be Sonic, Tails, Knuckles and Robotnik/Eggman. That would be a very complete set of characters for Sonic IMO, any other Sonic friends would not be well accepted. As far as other series go, I don't know much else about Sega, so I can't really say. I would say someone like Stryder could be a good addition though (Im sure there are others, but like I said, not a huge Sega fan other than Sonic, and the last one I played was Adventure 2, which was one of my favorite games of all time.)
Eh, I wouldn't expect them to buy back Rare, if anything, they'd probably just buy the IP's back.
They would probably only go for Banjo-Kazooie, which I would be okay with. It has so much potential to be another cash cow for Nintendo. Other series I would think they might go for would be Conker or Jet Force Gemini. But Conker would probably be passed up due to its mature content and similarities to BK, JFG would be pretty interesting to have back (Note: Sakurai had Lupus on his popularity polls for Brawl.) Could be some interesting stuff especially with the HD graphics and Gamepad. JFG has a lot of potential for improvements and, again, could possibly be another cash cow for Nintendo.
There are some good IPs from Rare that i sure do miss, but I don't really want them back. Perfect Dark comes to mind, and while it is a great shooter, it would fall into the generic FPS game, where most games lie nowadays. Also, I think a game like that is better suited for the Xbox, shooters dont do too well on Nintendo consoles and don't really fit the bill. With the game on Xbox 360, I think its best that it stays there. A new IP can be made for a shooter if Nintendo really wants it.