The Pebble Time and Time Steel have not been released and information on the interface etc. is sketchy.
Since the market is varied and limited, it seems logical to adopt a modular approach so that others can comment and adapt.
Any new application would need to have its own power supply in order to not affect the Pebble's selling point of long life. Thus the primary objective must be to develop a module to accommodate a reasonable power supply. This is not easy if the result is to be compact and attractive. Similarly the electronics to support power to the strap and to extend the life of the watch have differing requirements.
The basic SmartPack and PTS watch, as shown, is about 15mm thick. A thicker pack, of about 16.5mm, promises a battery of up to 200mAh. The wider version allows space for additional buttons etc.
A 3D print has been made to evaluate subjectively the effect of an extra thickness on the arm.
As a spin off it became apparent that a simple snap on bezel could be used. This would serve to protect the edges in a more 'sporting' environment.
My next response was to simplify and only consider self contained power to the strap. In addition, my current aim is to use a NATO strap; this means that an alternative charging method is needed encouraging the development of a backpanel.
I had already queried the viability of the Pebble/Xadow connector since it implied that there would be a 3mm single sided offset. The quest then became that of developing the thinnest back to accommodate a micro usb socket. The image shows a PT with a back which is 2mm thick, has a micro usb and accommodates a standard 22mm.strap. (Xadow type modules shown).
If an alternative plug type is used then the back panel could be reduced to say 1mm.
The models shown have been made in Rhino, rebuilt from the official models, as a proof of concept.