I want modules the same size as the portable computer that slide onto the front and the back of it. There should be no limit in the number of modules one can slide on either side.
On the screen side you can slide a lid, game controls, a qwerty keyboard that folds open or slides downwards, a set of speakers or an extra screen.
The batteries slide onto the back or the front. As many as I want to bring. The phone it self (since it doesn't work without its main battery) doesn't have to be entirely flat, it may have some bumps to accommodate bulky components. The back of the battery should be flat but the other side should of course have space for the bumps.
The batteries should also fit on the front side so that the weight is part of the keyboard.
Making phones as thin as possible is not self evident, I want to chose how much battery life I'm going to bring. Batteries just get old, the advertised and bench-marked time is for new devices. Charging not an interesting activity to me. The extra weight is not an issue. Actually, that it is so light and flimsy is annoying at times.
The attachments can have data or even whole computers in them. Publish a standard. In stead of apps you could buy physical attachments that you can swap. TB's of game assets, your entire music and movie library. All of your videos and photos. A pirate box seems fun or some other server. Have other phones connect to your wifi hotspot and play lan games without involvement of the mothership.
edit: Ideas are cheap, and with ideas I mean your ideas :P