If that's the case, then it's quite possible that a Pokemon like Keldeo can be caught in a place such as the Abyssal Ruins. The re-enhanced Dive Ball mechanics could help with this. Problem solved.New Pokemon in the new games? It's all but a guarantee. They just won't be new to US because we've been playing with and discussing them ever since BW came out.
Keldeo, Meloetta, some sort of extra forme for Kyurem, maybe even Genesect at some point. We all knew they were going to come out and assumed they'd be event Pokemon. But you know what? A sequel means that none of them have to be event Pokemon; they can simply be put in the new games as standard legendaries.
As for completely new Pokemon we've never seen before, that's pretty much impossible from a practical standpoint as has already been explained. Backwards compatibility would be totally screwed between generations, and it'd require them to code a whole load of new stuff. No, that's not going to happen.
As for R/S, I think it pretty much impossible that these new versions will be remakes of them, for business reasons (they'll make way more money selling stand-alone remakes of them). However, I don't think so farfetched that there will be some sort of Hoenn connection in these games, much like there was a G/S connection in FR/LG with the Sevii Islands. I think something along those lines is much more likely than the ability to explore the whole region G/S/C style, as the ability to do so would detract from the appeal of the (likely) upcoming remakes.
I do think there is SOME sort of connection to Ruby and Sapphire though; as people have pointed out, the title images for each of the games hints too strongly at them for there not to be. Even the lines inside the 2s match up! That's stretching coincidence pretty far.
Also, I don't see any reason why Hoenn teasers can't be a part of the game, but it's so unlikely that these will be Hoenn remakes. Putting the remakes on the 3DS in their entirety would be more of a penny pincher than conceiling them in two games titled otherwise. I know I would buy a 3DS down the road if I could replay my favourite generation, and I'm sure a lot of other fanboys would too. And it's ridiculous to think that GF and Nintendo actually care about making more money on a DS game than a 3DS game, especially when the DS is quickly becoming obselete.