I don't want to keep bringing up Ridley and K. Rool, but it really bears repeating as people keep regurgitating the same arguments. (I'm done talking about it though so this it from me.)
Ridley, if he is not in, is not in because Sakurai thinks he wouldn't be a good representation of himself as a playable character. He always show cases the character. He knows people LOVE Ridley, so if he isn't in there is a reason for it. It's not a time reason. It's not a "oh, Marshall needs to be in so lets not plan Ridley" reason. It goes beyond being "too big," I think Ridley would be an awesome playable character but I have seen other Metroid fans argue against him. He is portrayed in the games as, a quick, powerful, imposing figure. It would be hard to really capture that as a playable character. I don't agree with that, but I kind of understand it.
K. Rool you could argue is a victim of development time going to other characters, modes, etc. I would theorize that if he was planned he would be seen as a higher priority character, and that his relative inactivity would be a large part of his absence. You can say relevancy doesn't matter, but if he DOESN'T make it, it probably played a part.
Also, none of the currently announced character count as retro, as they have had games within the last 4 years and modern redesigns. Captain Falcon is more retro then Megaman or Little Mac at this point. There's no rule that we HAVE to get a retro rep, though. I'd really love to see Mach Rider, Duck Hunt Dog, or Takamaru, (or NES Ridley, he's retro AND Ridley, damn it!) but none of them HAVE to make it to meet some imaginary retro quota.