They missed their chance to make any major hardware changes when they released the 3DS XL. There's too many of them out there with only one circle pad for them to make a push towards 2 of them, as the rest of the customer base would be alienated. I can't see a "3DSi" type announcement until TGS.
I'd say wait until you see a good deal on it and pull the trigger, unless like me you can't wait till Fire Emblem is finally released in Europe and already got one.
Here's the thing, the DS was a bulky handheld that was poorly designed. They fixed that with the DS lite. The DSi responded to changing environments to allow for downloadable games. Then the XL just added a bigger screen for those who don't mind a large hand held. The 3DS is a nice sleek piece of hardware and the 3DS XL is the exact same. Nintendo only released updated DS' when there was an actual need to make fundamental changes to the hardware. Unless Nintendo decides to make a 3DS phone I don't see where they can improve on the 3DS' design and it seems too soon to just make a smaller version.
I personally don't think the 3ds will be a focus at E3 this year. The WiiU needs a real push; Nintendo will want their press conference leaving consumers excited about the WiiU's future and I suspect they won't want the 3ds to steal its thunder, as its already selling quite well.
That said, there's no harm in waiting it out (so long as you don't desperately want the Pikachu model).
I'm betting they'll wait at least until next (2014) E3 for a new 3DS model. Once a year is crazy, once every two years sounds fine. I'm not sure what they could add to the 3DS to warrant a third version, particularly so soon.
The DS Lite came out because the first DS was ugly and looked like a dev kit.
The DSi came out because they wanted to add new functionality to the DS that it wasn't capable of doing.
The DSi XL came out, because um... some people have big hands?
The 3DS XL seems like it addresses some of the build problems of the original 3DS (though they aren't anywhere near the original DS) and gives it a larger screen.
I see potential for a version with a second circle pad built in or towards the end of life, a more efficient processor/screen/etc to give better battery.
I don't really see what they could do to improve the design, besides add another analogue stick and I doubt that's something they'd do now.
No harm in waiting a few more months though, Animal Crossing is the first game coming out for the thing that feels like it really needs to be played on the bigger screens.