Nintendo clearly needs to have a dedicated line for 2D games, and put serious effort on it, as much as for 3D games.
What I'd like to see in these games is a darker theme, orchestral music (like the 3D games get, no lesser treatment), excellent sound effects, and especially tight gameplay. NSMB was fun for sure, but I'd like a Mario game paced a little more like the older ones, where running you went fast.
I also love the HD sprites idea and ADSL's post http://gamrconnect.vgchartz.com/post.php?id=4317640.
Yup, Nintendo needs to make more new 2D classics with high production values. I don't know about the darker theme though. For one, it's not necessary and two, it's going to put off more people than it would bring in.
I am not sure what you mean by "where running you went fast". I suppose you mean Mario stretching his arms out in SMB3 and SMW after running for a while. That didn't mean you were running faster though, just that you could lift off now, in case you had a tail or a cape.