While thinking about how many newcomers/veterans we may see before the release of the 3DS version, we should not forget that Smash Bros for 3DS is released in Japan on September 13th. So I would say any character reveal has to be before THAT date.
In general I expect a FE newcomer, a DK newcomer, maybe Shulk and Takamaru. However I see that because of the timetable we may only see 2 out of these 4 at maximum before the release of the 3DS version in Japan. Takamaru could easily be shown after release as he wouldn't create much more hype in my opinion. a DK newcomer(K.Rool, Dixie or Cranky) could get a certain attention, so I expect a pre-release reveal. Now with the FE newcomer, I'm not sure. There is a possibility Sakurai may show him before release as well as there is an option for a post-release reveal.
If I were to choose two newcomers out of these four I would go with Shulk and a DK newcomer.(most likely K.Rool)
Shulk is a good choice because Xenoblade Chronicles did so well in the west and a sequel is coming finally in 2015. K.Rool makes sense to create even more hype right before the release as he and Ridley are highly requested.
Another possibility would of course be to create trailers featuring two characters. (Shulk and Takamaru, Shulk or Tamaru and FE newcomer) Honestly I don't see something like this happen, but the possibility is given since the Greninja reveal trailer.