There will be another GBA - it will be as small as the micro, or maybe a little larger. It will have a very large screen compared to the size of the whole console; and will look like it will feel awkward in your hands. It of course will not, and will fit perfectly. I'll have a D-pad (no analog) 2 shoulder buttons, start, select, and x,y,a,b. It'll use the same gpu and cpu as the ds, but will not be reverse compatable - of course it will have built in wifi and a front end, not unlike the ds currently. The big difference people will be shocked to see is that it wont have cartridges, and you wont be able to buy cartridges for it in retail shops. Everything will be by download, and it'll have plethora of classic games (and some new ones - but mostly rely on the appeal of old games). There will however be a slot for an SD card, which much to the dismay of gamers everywhere will function exactly like the Wii's SD card, in that you can't actually run games off it. You will however be able to run MP3's off the SD card.
Since not very much shelf space will be needed for it - you'll start to see it pop up in places where you never expected to see a game system.
Places like convience stores, cell phone trollys in the centre of the mall, maybe even beside the chocolate bars as you check out at target.
This will supprt the DS even further as it'll get people to pick up video game systems on a whim, and as you paruse the online shop they can swiftly remind you that Wii Health Pack is now available on the Wii, as is Brain Training 2 on the DS Lite. I suspect a movement away from games that we enjoy on the most part (kirby's zelda's mario's etc) on the DS and have them on this new GB, as a touch screen or a second screen is not really needed for those types of games.
This thing will be cheap, less than $100, maybe even less than $80. It'll be announced next month at E3 (I think this is the big console announcement) and Reggie will also say they are hoping to have it ready for Christmas - which it will be - but the launch will feel rushed - and it will take a while to gain momentum - not unlike the DS did 3 years ago.