No offense, but there's no way Nintendo will announce a new handheld this E3. Not when they just released the DSi. They would be taking away many potential customers, because they'll think, "Oh, if a much much better handheld will be coming out next year, I might as well wait!" And Nintendo cares too much about its money to risk losing some (and who can really blame them, they're a business, and like most businesses, they want as much money as possible).
A new handheld might be announced next E3 however, because as you stated, the DS is already about 5 years old (and by then it'll be almost 6). But even that might be a stretch, especially if DSiware and DSi retail software really take off...
As for what I would like to be in a new handheld...well you guys gave me a whole bunch of ideas, so I'll steal some of them if you don't mind.
A 2mp camera (or better), both screens widened (why stop with one?), built in rumble (it's about time handhelds started having rumble built in), ability to connect to any hotspot a laptop can, motion sensing (I'd be fine without it tbh, but it'll be nice to have nevertheless), ability to take videos with your camera and add movies to your system, and...I can't think of anything else that I would really want. I'm pretty satisfied with my DSi actually.

EDIT: Oh, and an Internet Browser that can stream video and audio (or if that can be created as an update for the DSi's browser, even better!).