One thing what distinct one watch from another is a display. Everyone wants something unique that would stand out in a crowd. NycResistor have been playing around with HDSP211x alphanumeric display which is actually based on LED arrays but looks like it were a Nixie.
The problem is to make it small as possible it you want to call it watch. A Teensy 2.0 seemed to be a nice fit. Even display pins lines up with board – few wire jumpers had to be made because power pins weren’t in right position. So far it is working with software RTC and needs some battery backup and proper power supply.