I would absolutely love to see more 2D handhelds, after all, they are my favorite.
I fear a loss of 2D games in the future, and that really disappoints me. Though, we can't assume that they will toss them out the window - but it seems like they went with the production of GBA games. I think we should expect artistic styling to be diverse, I do not suspect that they will make all of the Legend of Zelda handhelds with cel-shading.
Of course, the further we progress, the better the newer systems can handle higher quality graphics. With this, I'd assume, Nintendo would experiment with numerous types of styles. I do not think an era has been lost, but rather, an old era will mesh with a new.